The core rules of d&d (mechanics but not settings) are available under a free license (it was the Open Gaming License, or OGL, although I have a feeling it’s a different license (Creative Commons , maybe?) now following the backlash from earlier this year when Wizards tried to scrap the OGL and replace it with a worse license)

Pathfinder was originally based on the d&d 3.5e ruleset. I’m not sure how far it has diverged from that, as I’ve played neither. Solasta is based on d&d 5e (the latest version of the rules), but is in a non-d&d setting of their own creation. BG3 is also based on 5e (although less strictly than Solasta), but also has the D&D license, so can use the Forgotten Realms/Sword Coast setting

Personally, I enjoyed Solasta, and think it’s a great representation of combat on 5e. The writing and story aren’t amazing, and it lacks the crazy amount of freedom/choices of BG3, but mechanically, it’s a great implementation of 5e rules.

Yeah. Probably not actually my favourite at the moment, but definitely the game I’ve sunk the most amount of hours into.

You’re undead, and use magic and guns to fight aliens and gods in defence of a big ball. The big bad is made of ghosts (but not the kind you’re friends with).

In the UK, yes (mostly). In our system, we’ve got further education (sixth form or college) which sits between high school and higher education (university). As well as providing A-level courses, colleges often provide more vocational courses that don’t necessarily lead into higher education. For example, my local college has hairdressing and bricklaying courses.

Confusingly, some universities are also made up of colleges. I think this is a minority of universities, though, and anecdotally, seems to be the older ones (Oxford, Cambridge, etc)