Is it possible to get the code from any app on the App Store and maybe change it in the Xcode?

>Is it possible to get the code from any app on the App Store

An app in the store is a compiled binary. It is not possible to obtain the code from a store download.

If it were, anyone could rob code from anyone else, so this is a good thing. It's disappointing that someone would even come here with such a request.

Devs are expected to write their own code for their own apps.

in addition, it is often illegal ; read the licence agreement you sign, most often they state that you are not authorized to decompile code.

So, forget it and do as KMT advised.