If you have Cross Save, your game will (also) be stored online. So every time you start any version of the game on any hardware you can sync to that and load your last savegame. You do need to own the game for every hardware.
The needed Versions are: PC-Version for Windows and Linux (including Steam Deck), PS5 for PS5 and so on, depending on what they release.
They say in a Steam Update:
"Baldur’s Gate 3 will also support cross-save progression between PC, Mac and PS5, allowing you to transfer your saves between platforms and continue your adventure wherever you purchase the game. Personally, we’ll be taking our experience on the road with the Steam Deck, and then venturing home to enjoy the cinematic experience of interrupting an ogre’s horizontal tango from the comfort of our sofa. Don’t forget the popcorn!"
Source:
https://store.steampowered.com/news/app/1086940/view/3640634026230039737?l=english