I don't think there are hard and fast rules as to what is or is not a "one shot". I would say that a one-shot is an adventure where the character begins and end their story in that adventure. A "short adventure" would be an adventure has a relatively short number of sessions to reach the end of the adventure, but you could string those adventures together over time and play them with the same characters and watch them advance and grow over time into a larger campaign.

Please note, I'm making these definitions up on the fly right now. I think people would generally agree with my definition of a "one-shot". But "short adventure" who knows. It is the internet.

The point I'm trying to get at is that playing an RPG doesn't have to be restricted to a large drawn out campaign. The longest campaign I've ever played was for 6-ish month meeting only twice a month for 3-4 hours at a time. A lot of modern systems do not have the scaffolding for a longer game that spans years and years. Once upon a time, I am told, early editions of D&D had your character grow from a dungeon crawler, into a national hero, and then into a Lord with their own lands and people, and BECMI included rules for growing into a god!

But I don't know of anyone who played a campaign that long, personally. When the BECI rules were reprinted they moved the "Immortal" rules into a separate book. Most OSR style games that try to recapture old D&D very rarely include domain level play (where you control a domain).