Operations that do not require compute resources are typically those that can leverage previously cached results. However, if no queries have been executed previously, all the given operations would require compute to execute. It's important to note that certain operations like DDL statements and queries that hit the result cache do not consume compute credits2.

External functions in Snowflake are a special type of User-Defined Function (UDF) that call code executed outside of Snowflake, typically through a remote service. Unlike traditional UDFs, external functions do not contain SQL code within Snowflake; instead, they interact with external services to process data2.
https://docs.snowflake.com/en/sql-reference/external-functions.html#:~:text=External%20functions%20are%20u

Snowflake external tables are read-only, which means data manipulation language (DML) operations like INSERT, RENAME, or ALTER cannot be performed on them. However, external tables can be used for query and join operations3.
References: [COF-C02] SnowPro Core Certification Exam Study Guide