Firstly you need a good file explorer to access the obb folder as a lot of file explorers on Android 11 will not allow you to access this folder as Google restricted access to it. My personal recommendation is MiXplorer. Download Balatans MiXplorer Silver from here. MiXplorer not only allows you access to the Android/obb folder but also the Android/data folder on your internal SD storage.
Depending on where you download a game from that has obb files, they can either come as xapk, apks or will be a separate file to the APK. If it comes as xapk just use a split APK installer. Normally I would recommend SAI app for this but it doesn't support xapk so use APK Mirror Installer instead or an app in here that Balatan uploads called App Manager which has an excellent package installer built in. Apks is fully supported in SAI and App Manager. If you download a game that has the obb file separate, what you need to do is create a folder in Android/obb with the same package name as the game. It usually starts with com. Once you create the folder, copy the obb file inside the newly created folder and then install the apk. Make sure you copy that obb file first before installing APK as a lot of games will think you haven't purchased the game or haven't downloaded from play store.
If there's anything you don't understand just ask.
