Demand...

How does the demand work?

So I've got 0 demand for homes. But commercial and industrial is full. BUT I have 50k open jobs.

My style of play is paint and make stuff up my head but decided I'm gonna give simulation a go and correct my city... I've redone my trains... that seems to bring population in... working on public transport next.

Also I don't understand simulation regards to schools etc... it says I have 18k eligible elementary... but only 11k spaces. Yet none of the schools are full and my coverage is all green. Why?!