What is the easiest way to add files to an apk file and to then install said apk file to a rooted android phone?
All SE answers I’ve seen so far either (a) don’t work on new versions of android/android-studio or (b) respect the android signing process
What is the easiest way to (while throwing security considerations into the wind) to run a modified apk?
APKs are, in the end, just glorified ZIP files, and any computer that can open a ZIP file can open an APK to add and remove files. Just rename it to .ZIP and work with it.
However, in order for Android to trust the APK and allow it to be installed, it needs to be signed. To sign it, you can follow the instructions in the answers to this SE post: How to Sign an Already Compiled Apk
You can change any APK you like, even one signed by a third party, but you cannot sign it with their key — since you don’t have it. That’s the point of signing. So you’ll have to sign it with your own.