My neighbor said that he acted as the general contractor when his current house was built. I'm not sure of all of his previous experience, but he was a millwright, and perhaps a manager at work.
In my case... well, let's start with my father who apparently helped his parents build their house when he was young. That house was a fairly modest box shaped house from the 1950's.
Then when I was young we built my parent's house. An extraordinary experience for two young boys.
Let's see. Custom designed with the help of a local architectural student (with previous commercial architect experience). I believe the county required an engineering analysis to approve the plans.
Excavation was contracted out. And septic tank was also contracted out.
We formed up the foundation, and I believe poured the foundation.
Hired out concrete block work to a "retired" mason for foundation walls, and an integral brick wall in the middle of the house. He did some excellent work.
Our previous neighbor was a licensed electrician. He did moonlighting work to help coordinate the electrical work. But, I did most of the pulling wires and wiring plugs and switches at age 13. An excellent experience.
I think we also did most of the plumbing.
We did hire the architect to help build the house during the summer. A unique arrangement. And, a few other "handyman" type laborers.
We did the sheetrocking, but hired out taping and spackling.
Kitchen cabinets were made by a local company.
My parents did pay worker's comp on all employees, and have continued to do that for any farm employees that they have.
Oh, and while we were building the house, my parents had the main barn structure contracted to be built.
Financing was mentioned earlier. I don't remember all of the details. The land was mortgaged, but I believe the house lot technically had to be paid off. My grandparents loaned my parents some of the money for building the house. And, at some point we moved into a pair of trailer houses on the property and sold the previous house to raise funds for the building.
Mom had a rule, we couldn't move into the house until it was done (or 99% done).
Overall it turned out very nice for self built houses.
In Oregon, most construction, electrical, and plumbing can be done by owner occupants.
I've done some remodeling and maintenance projects over the years.
I'm wanting to build a large "finished" workshop, although a little stuck getting started.
Many of the building codes are the same now as they were 50 years ago. However, one of the issues that I'm running into is there are a lot of new construction techniques. We did galvanized pipe in the barn and copper pipe in the house. Modern construction uses PEX which has quite a few advantages. Other fields such as insulation has evolved over the years.
Anyway, it would take a bit to keep on top of all the modern technologies.