Explanation
The annotation @Environment.systemField: #LANGUAGE is used to assign the ABAP system field sy-langu to an input parameter of a CDS view or a CDS table function. This enables the implicit parameter passing in Open SQL, which means that the value of sy-langu will be automatically passed to the CDS view without explicitly specifying it in the WHERE clause. This also applies to the CDS views that use the annotated CDS view as a data source, which means that the value of sy-langu will be propagated to the nested CDS views (view on view)12. For example:
The following code snippet defines a CDS view ZI_FLIGHT_TEXTS with an input parameter p_langu that is annotated with @Environment.systemField: #LANGUAGE:
define view ZI_FLIGHT_TEXTS with parameters p_langu : syst_langu @<Environment.systemField:
#LANGUAGE as select from sflight left outer join scarr on sflight.carrid = scarr.carrid left outer join stext on scarr.carrid = stext.carrid { sflight.carrid, sflight.connid, sflight.fldate, scarr.carrname, stext.text as carrtext } where stext.langu = :p_langu The following code snippet shows how to use the CDS view ZI_FLIGHT_TEXTS in ABAP without specifying the value of p_langu in the WHERE clause. The value of sy-langu will be automatically passed to the CDS view:
SELECT carrid, connid, fldate, carrname, carrtext FROM zi_flight_texts INTO TABLE @DATA(lt_flights).
The following code snippet shows how to use the CDS view ZI_FLIGHT_TEXTS in another CDS view ZI_FLIGHT_REPORT. The value of sy-langu will be automatically passed to the nested CDS view ZI_FLIGHT_TEXTS:
define view ZI_FLIGHT_REPORT with parameters p_langu : syst_langu @<Environment.systemField:
#LANGUAGE as select from zi_flight_texts(p_langu) { carrid, connid, fldate, carrname, carrtext, count(*) as flight_count } group by carrid, connid, fldate, carrname, carrtext The annotation @Environment.systemField: #LANGUAGE does not prevent the possibility of overriding the default value with a value of your own. You can still specify a different value for the input parameter p_langu in the WHERE clause, either in ABAP or in another CDS view. This will override the value of sy-langu and pass the specified value to the CDS view12. For example:
The following code snippet shows how to use the CDS view ZI_FLIGHT_TEXTS in ABAP with a specified value of p_langu in the WHERE clause. The value 'E' will be passed to the CDS view instead of the value of sy-langu:
SELECT carrid, connid, fldate, carrname, carrtext FROM zi_flight_texts WHERE p_langu = 'E' INTO TABLE @DATA(lt_flights).
The following code snippet shows how to use the CDS view ZI_FLIGHT_TEXTS in another CDS view ZI_FLIGHT_REPORT with a specified value of p_langu in the WHERE clause. The value 'E' will be passed to the nested CDS view ZI_FLIGHT_TEXTS instead of the value of sy-langu:
define view ZI_FLIGHT_REPORT with parameters p_langu : syst_langu @<Environment.systemField:
#LANGUAGE as select from zi_flight_texts(p_langu) { carrid, connid, fldate, carrname, carrtext, count(*) as flight_count } where p_langu = 'E' group by carrid, connid, fldate, carrname, carrtext References: 1: ABAP CDS - parameter_annot - ABAP Keyword Documentation - SAP Online Help 2: ABAP CDS - session_variable - ABAP Keyword Documentation - SAP Online Help

Explanation
The function module ZF1 is in a software component with the language version set to "ABAP Cloud". This means that it follows the ABAP Cloud Development Model, which requires the usage of public SAP APIs and extension points to access SAP functionality and data. These APIs and extension points are released by SAP and documented in the SAP API Business Hub1. Customer-created function modules are not part of the public SAP APIs and are not released for cloud development. Therefore, calling a function module directly from a class with the language version set to "Standard ABAP" is not allowed and will result in a syntax error.
However, there is a possible way to call a function module indirectly from a class with the language version set to "Standard ABAP":
Create a wrapper class or interface for the function module and release it for cloud development. A wrapper is a class or interface that encapsulates the function module and exposes its functionality through public methods or attributes. The wrapper must be created in a software component with the language version set to "ABAP Cloud" and must be marked as released for cloud development using the annotation @EndUserText.label. The wrapper can then be called from a class with the language version set to "Standard ABAP" using the public methods or attributes2.
For example, the following code snippet shows how to create a wrapper class for the function module ZF1 and call it from the class zcl_demo_class:
@EndUserText.label: 'Wrapper for ZF1' CLASS zcl_wrapper_zf1 DEFINITION PUBLIC FINAL CREATE PUBLIC. PUBLIC SECTION. CLASS-METHODS: call_zf1 IMPORTING iv_a TYPE i iv_b TYPE i EXPORTING ev_result TYPE i. ENDCLASS.
CLASS zcl_wrapper_zf1 IMPLEMENTATION. METHOD call_zf1. CALL FUNCTION 'ZF1' EXPORTING a = iv_a b = iv_b IMPORTING result = ev_result. ENDMETHOD. ENDCLASS.
CLASS zcl_demo_class DEFINITION. METHODS: m1. ENDCLASS.
CLASS zcl_demo_class IMPLEMENTATION. METHOD m1. DATA(lv_result) =
zcl_wrapper_zf1=>call_zf1( iv_a = 2 iv_b = 3 ). WRITE: / lv_result. ENDMETHOD. ENDCLASS.
The output of this code is:
5
References: 1: SAP API Business Hub 2: Creating an ABAP Cloud Project | SAP Help Portal

Explanation
The nested join is evaluated from the top to the bottom in the order of the ON conditions. This means that the join expression is formed by assigning each ON condition to the directly preceding JOIN from left to right.
The join expression can be parenthesized implicitly or explicitly to show the order of evaluation. In this case, the implicit parentheses are as follows:
SELECT * FROM (a INNER JOIN (b INNER JOIN c ON b~c = c~c) ON a~b = b~b) This means that the first join expression is b INNER JOIN c ON b~c = c~c, which joins the columns of tables b and c based on the condition that b~c equals c~c. The second join expression is a INNER JOIN (b INNER JOIN c ON b~c = c~c) ON a~b = b~b, which joins the columns of table a and the result of the first join expression based on the condition that a~b equals b~b. The final result set contains all combinations of rows from tables a, b, and c that satisfy both join conditions.
References: 1: SELECT, FROM JOIN - ABAP Keyword Documentation - SAP Online Help

Explanation
A metadata extension is a RESTful Application Programming object that can be used to organize the display of fields in an app. A metadata extension is a CDS view that annotates another CDS view with UI annotations, such as labels, icons, or facets. These annotations define how the data should be presented in the app, such as which fields should be shown on the object page, which fields should be editable, or which fields should be used for filtering or sorting. A metadata extension can also be used to add custom actions or validations to the app12. References: 1: Refine the Object Page with Annotations | SAP Tutorials 2: ABAP RAP : Enabling custom actions with a dialog for additional input fields | SAP Blogs

Explanation
In class ZCL_CLASS_A, you use the statement DATA var TYPE *** to declare a data object named var with a data type specified by ***. The data type can be any of the following1:
A predefined ABAP type, such as i, f, c, string, xstring, and so on.
A data element from the ABAP Dictionary, such as matnr, carrid, bukrs, and so on. A data element defines the semantic and technical attributes of a data field, such as the domain, the length, the data type, the description, and the value range2.
A domain from the ABAP Dictionary, such as matnr_d, carrid_d, bukrs_d, and so on. A domain defines the technical attributes of a data field, such as the data type, the length, the output length, the number of decimal places, and the value range3.
A type defined globally in a class, an interface, or a type pool, such as zcl_class_b=>type_a, zif_interface_c=>type_b, ztype_pool_d=>type_c, and so on. A global type is a type that is defined in a global repository object and can be used in any program or class4.
A type defined locally in the current class, such as type_a, type_b, type_c, and so on. A local type is a type that is defined in the declaration part of a class and can only be used within the class5.
Therefore, the possible values for *** are B. the name of a data element from the ABAP Dictionary and D. the name of a domain from the ABAP Dictionary. The other options are not valid because:
A: The name of a type defined privately in class ZCL_CLASS_A is a local type and cannot be used with the DATA statement. A local type can only be used with the TYPES statement5.
C: The name of a type defined privately in another class is a private type and cannot be accessed from outside the class. A private type can only be used within the class that defines it.
References: 1: DATA - ABAP Keyword Documentation 2: Data Elements - ABAP Dictionary - SAP Online Help 3: Domains - ABAP Dictionary - SAP Online Help 4: Global Types - ABAP Keyword Documentation 5:
Local Types - ABAP Keyword Documentation : Private Types - ABAP Keyword Documentation