The official way to use git is by repeatedly calling git checkout, git diff, git rebase to view, compare, & copy bits of branches.  The way git is really used is by copying the repository to multiple directories, each viewing a different branch, so you can easily move bits between different branches & have simultaneous builds.  The mane problem is getting the differences between the 2 branches, along with files which exist in only 1 branch.  You  want to do that  for only source code files, not .dex, .git, .plist, or .png files.  The internet  struggled to do with this one with scripts, too, so uncle Lion made a simple solution in C.

https://cdn.hackaday.io/files/1380505906331200/diffdir.c

diffdir my_repo idiot_repo java kt

compares the .java & .kt files in the 2 repositories.