This article includes sales estimates for different handhelds from market research firm IDC.
They place total handheld PC sales of the Steam Deck, RoG Ally, Lenovo Legion Go, and MSI Claw at almost 6 million units for the past 3 years. It’s estimated that the Steam Deck makes up between 3.7 to 4 million of those sales, more than all the other major handheld PC manufacturers combined.
You can play those Switch games plus a ton of other games on the Deck. Apart from technical capability there are 0 restrictions on what you can do with your Deck. You can play Steam games, GOG games, Epic games, Nintendo games, PlayStation games, Commodore 64 games, Arcade games, etc.