This is what it was like when I was at home.
When my eldest daughter was a toddler, I watched a distant cousin's similar aged son a few mornings a week. The extra cash was welcome and it helped her out with some flexible care while she was at her 9 to 5 job and her dh did shiftwork in a factory.
When my kids were a bit older, I had a friend's two kids ft, and essentially they were just thrown in with my own kids. Their parents both worked 9 to 5. But I wouldn't have done it had I needed to be like a proper day care with a program, or staying home all day. They came on errands, did swim lessons, everything that we all did.
For a short period of a few months, as a favour to a friend in a tricky employment shift, I had her older kids, and taught them school. Would never have done that for too long.
I did a few research contracts when my kids were older. We home educated then, so I worked in the evenings mainly. It was a little extra cash in hand to buy a new fridge or something like that, I couldn't have sustained it long term.
When my oldest was old enough to babysit a bit, I took a job 2 hours at lunch time tutoring. That was closest to a "real" job, but had been incompatible, I'd have left the job.
After that I had no job for a bit, and then another with a few hours a week while kids were at school, and then I went back to work in a regular job, and had to make childcare arrangements. Then my dh retired and he is the SAHP who takes care of all things at home so he can do pick ups and drive kids around as needed. Though he does a little consulting on the side from time to time.
It's not cut and dried but it's pretty clear to me where the line was in both cases between sahp and wp.