External synth setup , Cubase 10.5?

I run Cubase 10.5 . I have 3 synths , Novation Mininova , Roland Gaia , and a Roland Aira System 1. I have had only partial success geting them fully integrated with Cubase, despite trawling the interweb and following the info presented . Issues I am having , include - but not exclusively by any means , ;
1- Not sure how to define the external hardware in midimanager ,
2- not sure what connexions need to be made ,physically or virtually,
can hear sound sometimes, sometimes records midi data, sometimes not.
3- if midi data is recorded , it does not play back any sound, from the external instrument. Sometimes the program seems to ‘force’ another VST into the inspector and uses that . Mostly Halion.
4- With the Gaia , what seems to happen is that the ‘forced’ VST plays, but also a G piano sound plays at the same time . Can’t work that out, although this morning I found out how to turn off the GM sounds on the Gaia. Thy’re crap anyway . You spend all tha filthy money on VSTs , only to use GM ?
5- I am using a Behringer UMC 404HD as my interface. Can anyone offer a simple walkthrough on how to make physical connexions and set up the midi within Cubase? I had kind of been led to believe all would take place over USB. Naieve , I know.All the appropiate drivers and such are installed.
Got me well stumped , beer only helps a little with this !!!