How can I change the inside app design?

How do I make the apps look different afterwards? For example, the app design from Huawei, Apple, Samsung, etc! I don't mean icons, but what the app looks like when it is running.

Either you download a custom rom, then everything is different, really everything.

Or for the icons a launcher with an icon pack.

Addendum:

but if you want to have a different app design (e.g. Settings) you have to install a custom rom

This is only possible with the source code and experience in programming apps e.g. With Kotlin and Swift. Or you can reprogram the app. Some apps may have an API to which you can connect your new app.

The colors of the surfaces can be changed with themes. The arrangement of the buttons and the structure of the app can't be changed.

Root is sometimes required for the required theme engine.