The Steam Deck comes with four different memory sizes in two models - the Steam Deck LCD and the Steam Deck OLED. If the memory is no longer sufficient at some point, you have two options for expanding your Steam Deck memory.
How much memory does the Steam Deck have?
The Steam Deck LCD is available in 64 GB, 256 GB and 512 GB. The Steam Deck OLED is available in two versions with 512 GB or 1 TB. These models already offer a fairly solid storage capacity for most applications and games.
The 64 GB variant uses eMMC memory, while the 256 GB, 512 GB and 1 TB models use the faster NVMe SSD technology.
Expand the Steam Deck’s storage space with a microSD card
To expand the storage space of your Steam Deck, all you need is a microSD card, which you can buy relatively cheaply from your trusted retailer. The Steam Deck has a microSD card slot on the "bottom right" (on the bottom) in which you can use memory cards of up to 2 TB thanks to the SDXC standard.
High-quality SD cards show little difference to internal memory in terms of loading times and loading textures. Only the writing speed of the SD card is slightly slower, but this is only relevant when transferring data and has no impact on the games themselves.
After inserting the microSD card, you can go to the storage settings and specify that games and applications should be stored on the microSD card.
Installing a larger hard drive in the Steam Deck - M.2 slot for a new SSD
If you're up for it, you can buy a suitable SSD in the M.2-2230 form factor and install it yourself. Installation isn't as easy as inserting a microSD card, as it requires opening your Steam Deck, but replacing the SSD isn't rocket science either.
Valve itself has provided a video here in which they demonstrate how to open the Steam Deck.