What are the rules for creating design in design called?

Question

User interface

In design, there are certain rules by which the design of the site is created, or the design of a mobile application or business card, here are the names of these very rules by which you can best create a design. For example, in the design of a mobile application, for example, you need to make this font under the text in 8px, and this button 15px, or in the design of the site, you need to make this title in 40 px, etc. I hope you understand what I am trying to convey. Thank you!

Answer 1

Guides, guidelines, guidelines. For example, Apple calls it the Apple Human Interface Guidelines.
Android has Material Design Guidelines.

Answer 2

...UI kit

Answer 3

There are principles. It's better not to look for rules. Each project is unique (at least it should be) and following something unified simply will not work.
Or use guides (iOS, Android).