How do social media advertising monitoring services work?

Question

Marketing

How do they collect such data? You can specify gender, age, social networks, device types. They provide ad variations for all advertisers, analyze impressions, engagement, and more.
For example, publer, adlover and the like
. And why do social networks themselves in the advertising account not allow you to see this.
By the way, Facebook has an official Advertising Library, where you can partially see such information, but there is no such detailed data there either.

Answer 1

You create a bunch of accounts and monitor ads from them

Answer 2

I also thought about a bunch of accounts, but it seems to me that there should be a lot of them in order to collect so much data. Maybe a browser extension, people install and don't know what data is being collected, don't they?