You are using words that have a specific meaning in this context - and you don’t seem to know understand.
The ‘flow’ and ‘return’ form a continuous loop via the various radiators and boiler. They are connected to one another so in one sense not separate at all- don’t forget it is the same water running throughout , not two separate loops…
If you are just replacing a radiator, you can copy what was there before, presuming it worked. If anything else it would probably be sensible to get a plumber in to help - simple/inexpensive job.