I share my PC with my brother. So, I was wondering if there was any way to lock certain apps. Making another account or a guest account is out of the question. Is there any app to do so?
In the system settings you can go to "Screen Time & Limits" (on the bottom, yellow box with parent with kid symbol) and then add the applications the current user shall not be able to open. It might not be comfortable to use if you regularly switch using the PC with your brother.
Honestly I still would recommend giving your brother his own profile and restrict the apps there. You could clone your profile if you want to save the hassle to manually copy stuff the other profile also needs.