So the android phones applications can be decompiled and source code is accessible through APK file. Then anyone will perform reverse engineering and use it to his advantage right?

Scenario 1: Let’s take the example of Yo*tuber he wants to start the channel, it not running successfully so he can reverse engineer the apk and get the algorithm to work in his favour ( I believe it is not possible because the main algorithm is saved/running in servers not in apk files , apk files are just a medium to view or create videos and upload to servers ) right??

Scenario 2: Let’s use example of dating apps or the famous app Tind . One can reverse engineer and do necessary changes ( here also main algorithm will not be in apk , but things like premium membership and paid benifits can be reaped and compiled as modified apk ) thats how the luckpatcher worked right and thats how the modded APKs are made? But why not save these details also along with algorithm so people can’t access it ??