Change Ubuntu design?

I heard that Ubuntu (desktop version) is an open source operating system. That's why I asked myself if it was possible such as To create a kind of texture pack in Minecraft or to change the icons and co in Ubuntu in some other way, is that possible?

You have to distinguish between the operating system and the software that you run on it. The fact that Ubuntu is open source does not mean that you can adapt any (closed source) software.

You can install any desktop environment under Ubuntu, you can imagine it like the launcher on Android smartphones.

From where can you install the various types of "launcher"?

From the package sources.

https://www.tutonaut.de/anleitung-linux-desktop-umgebung-aendern/