Everyone, until recently, signed all the trains. You could work a UTU one week, and NMT the next.
Now, there are three basic "links" I suppose you could call them - UTU/SGT, NMT, and TRV (Track Recording Vehicles plus MENTOR and Radio Survey). They are worked through choice, no seniority or anything like you would find at a TOC or FOC. The system was changed because crews operating knowledge of trains and their systems was being affected; you'd learn a train to death initially, then another, then go back to the first one and forget how everything works because you spent too long away from it. The systems are all fairly complex and have odd quirks, so it was better to simplify things.
Route knowledge is extensive but broad. We don't know any specific routes in depth like a driver, but have a huge overview of pretty much the entire network. We end up knowing exactly where individual mileposts are for good synchronisation and where fault hotspots are.