Possible to do solo with the right tools/enough muscle and stubbornness, much easier with 2 people

You don't need tons of concrete to build a foundation. It's better but not necessary. I built three foundations on my own so far with a concrete footer and pressure treated crawl space. Mixing, hauling, pouring and setting small batches of concrete on your own is possible.

A good and experienced builder can install drywall on their own. I'm not the best but I just finished drywalling my small porch on my own .... ceiling and walls. It's doable, not easy, not fun but it's possible.

It is better with two or more people to do these things but with modern building techniques, equipment and a bit of knowledge, just about every step is doable with one person.

It's not ideal, it is dangerous but building an entire house is doable by one person. It's also better based on age. In my 20s I had a hard time because I had the strength but not the experience. In my 30s I thought nothing of it all and did my best work ... now in my late 40s I'm struggling to do all these things now and seldom do anything on my own any more.

Except in order to be legal you need a licensed person to do much of the work. Licensed electrician to do electrical, licensed plumber to do plumbing, etc.