Depending on the phone, open settings,notifications, toggles all off, then just turn on the ones you want.
I recommend you look around all the settings and familiarize yourself as this is a basic process. Good luck
I found that if I disable all, I cannot enable specific ones, only again all of them. I don't want to try again because I have currently disabled a lot of the apps already and don't want to risk resetting them.
I have been using Android for years. I never did find a way to disable notifications by default or for all apps except a few.
I tried disabling notifications for every single app, but the screen keeps scrolling back to the beginning after each app and it became difficult to keep track of which app I had already done, not even considering the time required to do this task.