Neither of us is a builder by trade, no: I'm in software engineering, he was head of a large multinational organisation. He's a good joiner, and can plumb a bit, but when I wired up the house it was the first real electrical work I'd done, for instance (yes, it passed inspection!)
We got a few builders in for a few days to erect the roof trusses: I didn't fancy learning how to use a crane on that job, but we've done it on single story jobs with is and a couple of workies in to hold and nail things where told (and once by ourselves for a stick build job we did).
Most jobs are easy with a bit of time, especially internal framing, wiring, plumbing the heating etc. I hired a plasterer after doing one room, though: he did a better job in three days than I did in three weeks in that one room.
I also hired a roofer, with me acting as his labourer. We've tiled roofs on other builds in the past, but this roofline was a bit more complex.
In summary, especially for flat pack work, you can totally do it, but do find a good labourer to help you and some jobs are really worth a couple of grand for an expert.
This book is great, I've now got three editions of it:
The Housebuilder's Bible 13 (The Housebuilder's Bible: 13th edition) https://www.amazon.co.uk/dp/1916016804/ref=cm_sw_r_apan_i_S2CF2EG4Z2DVBMDJ02WF