The soundtrack is found by pressing Select on the game in the PS5 game menu and changing the game version to soundtrack.
The OST launches as a separate game, meaning it will close your current running game! Beware that so you aren’t forced to abandon your current run of Returnal!
Keep in mind, you are unable to transfer the soundtrack from your PS5 to an external device, like a USB, but the OST is also found on Spotify and Apple Music/iTunes.