This is the million-dollar question. Because the mod alters proprietary .pak files, it falls into a legal and security gray area.
A: Do not do this. Logging into Facebook with a modded APK that gives you infinite gems will trigger an automatic permanent ban on your social account. Plants vs Zombies 2 OFL PAK MOD
To understand the mod, you must first understand the jargon. This is the million-dollar question