Quite a bit to cover here, but I'll do my best.
Background: I'm an industrial trained electrician coming up on 8 years in the trade. In Australia we have to do a full 4-year apprenticeship to be trade qualified, so some aspects of my training will be quite different to the UK.
Like do you take all 3 phases to each outlet or can you split them?
That depends on the type of outlet. Installations with 3-phase power still have a neutral available, so for single-phase outlets you would run a circuit from the board that picked up one of the active phases plus the neutral.
3-phase outlets will use all three actives, sometimes without the neutral and sometimes with, depending on the type of load being supplied.
3 phase stuff confuses me a bit as there always seems to be extra parts, bigger stuff, converters and isolators and all sorts of funky stuff.
At its core 3-phase uses all the same control gear as single, just three times
The nature of things is that if a system needs 3-phase power, it's more likely to be big and complicated to begin with. We use all the extra funky stuff to do interesting things with electrickery.
How do people learn about it if they're never taught it? Is it very much a thing where you have to have a job doing it to be in the know...
This is the crux of the issue when trying to 'move up' in the trade. Industrial sparkies love to lay s*** on "house bashers" because part of our ego is that we work with the more complex gear and systems.
If you don't learn it as part of your training, it can be really hard to get a break into this side of the business, and no amount of book learning can match live training.
If you're genuinely interested in getting some experience and learning that side of things, my best suggestion would be to find some of the guys on here or a local business that works industrial and ask to help out. I'd certainly be more than happy to ramble on to a TA while we're doing a cable pull, and more than once I've taken time at the end of a day to run an apprentice or helper through the line we've been working on and explain things.