Starfield has a New Game+ mode which lets you replay it while holding onto some of your character's gains in the first playthrough. The specific details of Starfield's New Game+ options are spoilery, because they're justified by certain story details, but if you just want to know what you can do after the main quest is over, that can be revealed without going into the plot.
Still, you should back away now if you don't want any hints about the direction of Starfield's story or its conclusion. With that out of the way, here's the deal: After completing the final quest of Starfield's main story, One Giant Leap, you can...
- Start the game over with the same character, keeping their skills and powers
- Do that, but skip the main quest in the new playthrough
- Keep playing from your current save, with the option to start a New Game+ later
Starting a New Game+ playthrough will reset things like companion affinity, faction standings, and side quest completion, and you'll lose all your stuff—just skills and powers are retained.
You do get some new stuff, though: the Starborn Spacesuit Astra and Starborn Guardian ship.
Personally, I can't play an RPG without restarting with a new character at least three times, so I can't imagine replaying a giant RPG like Starfield with the character I've already been using for 100 hours. But on paper I like that you can keep your skills and powers and reset everything except the main quest, letting you experiment with other side choices and activities without having to retread the main story. (And you can change your character's appearance in Starfield by finding a plastic surgeon, if you get bored of looking at the same old mug.)
Saying more about these options would spoil aspects of Starfield's story you might want to discover on your own, so go forth into the main quest to see for yourself what happens, with the confidence that you'll be able to finish all the side quests you've left undone after the fact.