They’re often fixed together, depends on how heavy they are – a bigger heavier one isn’t going to wander about. Even if they are ones that fix together they’ll still be delivered in sections (otherwise they’d never get through the door) so you can still unfix them if you want.
Sometimes they’re in two parts, sometimes in three, with the 90 deg corner being its own small unit and two straights either side. In that latter instance you would maybe able to find the corner bit in varying angles, but probably as a custom option rather than off the peg, but if not, and you wanted to open up the angle a bit then one in 3 segments means you’ll at least have two smaller gaps rather than one big one