Icons next to categories

What you are looking for is Font Awesome 5 icons, as the description says: Find the Perfect Icon for Your Project in Font Awesome 5 | Font Awesome

Any icon you use in the category icon list setting, you add to svg_icons.
For example:

What you see:

Remember to provide the category slug, not the full name!
You can see the slug in the URL or the category setting.

I completely understand! This functionality is not part of the core, and a way to customize Discourse is by installing Theme components. This guide might help you to understand better: