How to start developing a mobile version of the site for DRUPAL8?

Question

Drupal

The main version of the site is already there, there are layouts for mobile. You need to create a mobile version of the site. How it's done?
Clearly not a browser check and if/else, right? Perhaps there is some module that uses a different theme for mobile phones, or I misunderstand the process...

Answer 1

Use responsive layout in the same theme. Of course, you can look towards https://www.drupal.org/project/mobile_detect, but I do not advise you.

Answer 2

If the theme is radically different, then create a separate theme and switch to it using the mobile_switch, mobile_theme or themekey modules.
If the theme just needs to be adapted to different screen resolutions, then google about adaptive layout, media queries, and more.