This is one of those questions nobody answers until the game is out, and Rockstar has not answered it. What follows is the pattern the studio has used for two console generations, which is the most reliable guide available.
How the last two Rockstar open worlds handled it
- Autosave triggered on mission completion and at a few other checkpoints, overwriting a single dedicated slot
- Manual saves available from the pause menu into a set of numbered slots, usable almost anywhere outside a mission
- Sleeping at a safehouse as an in-fiction save action in GTA V
- Mission checkpoints during missions themselves, separate from the save file
Red Dead 2 used the same structure with a larger number of manual slots. There is no reason to expect GTA 6 to depart from it.
Cloud saves
Both launch consoles back up save data to the cloud at the platform level. On PlayStation that requires an active PS Plus subscription; on Xbox it is included with the console. This works regardless of what the game itself supports, which means a dead console does not automatically mean a lost playthrough.
Rockstar has separately used its own social cloud saves in past titles, allowing a file to move between platforms in some cases. Whether that returns has not been stated.
Two playable characters, one save?
GTA V kept Michael, Franklin and Trevor in a single shared save with a switching mechanic. Jason and Lucia are shown working as a pair rather than as separate lives, so a single unified save is the obvious expectation - but the switching design has not been explained, and it is possible it works differently.
Practical advice for launch
Check that cloud backup is enabled on your console before 19 November. It takes thirty seconds and it is the only save-related thing you can actually control right now.
Launch timing details are on the release date page.