Gabe Newell, President and Co-Founder of Valve, answered user’s questions on Reddit, including questions surrounding the SSD that is under the hood — the M.2 2230 SSD. The M.2 2230 SSD has a small form factor but is not readily available to be purchased alone.

Source: N4G PC Steam Deck Utilizes Standard M.2 2230 Slot For Its SSD And Appears To Be User Replaceable