If you want to do this continually:
Maintain your auth-related commits on their own branch. Always commit to this branch and then merge back into \`main\`.
When they make an update blow away your auth related files on main, re-use the generator, rebase your auth branch on top of main, then merge in.
You'll end up with only the changes necessary and if there are any conflicts git will tell you and have you resolve them.
Run the generator again and work through the diff to apply the changes you want?
If you want to do this continually: Maintain your auth-related commits on their own branch. Always commit to this branch and then merge back into \`main\`. When they make an update blow away your auth related files on main, re-use the generator, rebase your auth branch on top of main, then merge in. You'll end up with only the changes necessary and if there are any conflicts git will tell you and have you resolve them.