Let me put it this way--after doing this job you will swear that you will never get involved in doing another. I last did a "turbo" job years ago when I worked full time in friend's Euro car indy shop. Back then the reason for the jobs was to replace worn out turbos so the "bigness" of the job seemed worthwhile as the end result was "big". In my most recent case my 245Ti came first. A week before we took it off the road 2 years ago (replaced by an incredibly low mileage 2002 S40--wife's call) it blew out one gasket. I was reluctant to abandon the 245 (loved driving it) and held onto it until recently when I learned of a path to get the title for the '93 245 I had transferred my Oldsmobile 215 V8 to. It was a hard decision to make and obviously the gasket would have to be fixed to make the car saleable.
So--to answer your question. The turbo stays with the manifold since there's no way to get at the bolts to release it. There are two difficult areas on the B21FT. The heat shield has to come off first - a convoluted mess of stamped steel pieces with bolts and/or studs from the top and bottom. Next is the "T" bracket at the bottom of the manifold-held by an 8mm stud. With that nut and the "manifold to head" nuts removed you can't tilt the works enough to pull it all back enough to slip new gaskets in place. The two bolts holding the "T" bracket to the block have to come off as well. All those bolts and studs underneath are hard to reach and leave little room to swing a wrench. You have to get mad--and use lots of naughty words. Children should be removed from the area.
Of course after enduring all that, a long time service customer came to me with his '88 Bertone B230FT - blown rear cylinder exhaust gasket due to a broken stud. Reluctantly I agreed to do the job--my first with a B230FT (also had the Garret-but water cooled). On the B230FT the heat shields are separate pieces so that part was easier. Underneath was a different story as the remote filter housing is totally in the way of getting to the "T" bracket bolts to the block. I lucked out in that the stud for the "T" bracket to the manifold unscrewed which allowed me to pull the manifold back (had to remove the whole works altogether as the second to last "manifold to head" stud was broken and had to be drilled out). With the "T" bracket stud removed you can lift the works off the motor.
If you are lucky too, that stud will come out on your B21FT. The two bolts from "T" bracket to block are very tricky to get started on re-assembly. Good luck. -- Dave