What is the tool for checking the adaptability of the site, by user-agent?

Question

Software testing

I need a tool, preferably one that works locally, to test the adaptation of the site to mobile devices.
Having searched the Internet, I found many services and extensions for browsers (it is already installed in firefox), but they all work
on the principle of resizing the browser window, which in some cases does not help. For example, if using them to go
to mail.ru, habrahabr.ru and other sites, no adaptability is visible, not like if you go from the phone. Because the user-agent is not transmitted, probably.

Answer 1

In the browser debugger (for example, chrome) there is a mobile mode button where you can select a specific device, a specific screen resolution. After choosing to update and that's it. If I understand the question correctly.