If you want to stream games from your PC to the Steam Deck using RemotePlay, you need to follow the steps mentioned in this post.

Whichever model you have, buying a microSD card for extra game ... If you envision going into the Steam Deck’s desktop mode, you may want to grab a compact wireless keyboard to type on.

and move files between the internal drive and microSD card. How To Check Steam Deck Storage In Desktop Mode Interestingly, you can also check the storage space in Desktop Mode on your Steam Deck.