-sA (TCP ACK scan)
This scan is different than the others discussed so far in that it never determines open (or even open|filtered) ports. It is used to map out firewall rulesets, determining whether they are stateful or not and which ports are filtered.
The ACK scan probe packet has only the ACK flag set (unless you use --scanflags). When scanning unfiltered systems, open and closed ports will both return a RST packet. Nmap then labels them as unfiltered, meaning that they are reachable by the ACK packet, but whether they are open or closed is undetermined. Ports that don't respond, or send certain ICMP error messages back (type 3, code 0, 1, 2, 3, 9, 10, or 13), are labeled filtered.
https://nmap.org/book/man-port-scanning-techniques.html

A thin Whois model is a type of data model that is used by some domain registrars for storing and looking up Whois information. In a thin Whois model, the registrar only stores the basic information about the domain, such as the domain name, the registrar name, the name servers, and the registration and expiration dates. The rest of the information, such as the contact details of the domain owner, the administrative contact, and the technical contact, is stored by the registry that manages the top-level domain (TLD) of the domain. For example, the registry for .com and .net domains is Verisign, and the registry for .org domains is Public Interest Registry. When a Whois lookup is performed on a domain that uses a thin Whois model, the registrar's Whois server only returns the basic information and refers the query to the registry's Whois server for the complete information1.
As a hacker, if you are unable to gather complete Whois information from the registrar for a particular set of data, it might be because the domain's registrar is using a thin Whois model and the registry's Whois server is not responding or providing the information. This could be due to various reasons, such as network issues, server errors, rate limits, privacy policies, or legal restrictions. Therefore, the probable data model being utilized by the domain's registrar for storing and looking up Whois information is a thin Whois model working correctly.
References:
* Differences Between Thin WHOIS vs Thick WHOIS - OpenSRS Help & Support

Information Gathering from Robots.txt File A website owner creates a robots.txt file to list the files or directories a web crawler should index for providing search results. Poorly written robots.txt files can cause the complete indexing of website files and directories. If confidential files and directories are indexed, an attacker may easily obtain information such as passwords, email addresses, hidden links, and membership areas. If the owner of the target website writes the robots.txt file without allowing the indexing of restricted pages for providing search results, an attacker can still view the robots.txt file of the site to discover restricted files and then view them to gather information. An attacker types URL/robots.txt in the address bar of a browser to view the target website's robots.txt file. An attacker can also download the robots.txt file of a target website using the Wget tool. Certified Ethical Hacker(CEH) Version 11 pg 1650

Banner grabbing is a technique wont to gain info about a computer system on a network and the services running on its open ports. administrators will use this to take inventory of the systems and services on their network. However, an to find will use banner grabbing so as to search out network hosts that are running versions of applications and operating systems with known exploits.
Some samples of service ports used for banner grabbing are those used by Hyper Text Transfer Protocol (HTTP), File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP); ports 80, 21, and 25 severally. Tools normally used to perform banner grabbing are Telnet, nmap and Netcat.
For example, one may establish a connection to a target internet server using Netcat, then send an HTTP request. The response can usually contain info about the service running on the host:
Graphical user interface, text, application Description automatically generated

This information may be used by an administrator to catalog this system, or by an intruder to narrow down a list of applicable exploits.To prevent this, network administrators should restrict access to services on their networks and shut down unused or unnecessary services running on network hosts. Shodan is a search engine for banners grabbed from portscanning the Internet.