How are responsive websites tested?

Question

css

Good day to all!.
How are responsive websites tested? Online services? maybe it's some kind of emulation of mobile phones? I don’t quite understand how to be sure of the correct display of my layout on mobile phones (after all, just changing the permissions is not enough, as far as I understand?!). I use it myself:
quirktools.com/screenfly
beta.screenqueri.es
www.responsinator.com
Is that enough? Of course, the most reliable way is real mobile phones, but there are not so many of them.

Answer 1

1. Firefox -> Menu -> Develop -> Responsive Design
2. Safari on Mac -> Responsive Design Mode

Answer 2

https://www.browserstack.com/
https://developer.chrome.com/devtools/docs/device-mode

Answer 3

FireFox: Ctrl+Shift+M and drag the corner in different directions and see how everything works at different resolutions.