Value has posted a video on disassembling the Steam Deck. The main idea was to replace a damaged thumb control or the M.2 2230 SSD.
The Steam Deck is not really designed to be repaired easily. There is a microSD card slot which can expand storage.
Most of the games in the library here would melt the Steam Deck into your lap. They are far too demanding and even an RTX 2080 works up a sweat.
The Steam Deck is fine for easy system requirement games like Clicker Heroes.