You need to Sign up with the account on which external drive game are registered with. Next you need to set path other these game destination folder to play the game.
Yes, I believe S21 is good option even in 2022. it has all the latest specs with future proof hardware. This phone is also getting Android 13 with UI 5.1 support.
Yes I think Asus is a good option, but I recommend going with 12th gen Laptop, since you would like to play AAA titles. My suggestion would be like "HP Victus Gaming Laptop 15-FA0032DX" or relevant With GPU min 3050 TI