Think of the APK as the "installer" or the "program file." It contains the code, resources, assets, and manifest of the application. When you download Tomb Raider from a third-party source, the .apk file is the first thing you need. It tells your phone how to set up the app and where to look for data.
The OBB file is the heavy lifter. In Tomb Raider , the OBB contains: tomb raider android apk obb