I live in Sweden with my nearly three year old kid, who is at preschool at the moment. The system here is really good, and is based around encouraging parents to be able to go back to work/study if they want. If one parent at home and not working or studying you will be limited to 15 or 30 hours of preschool per week.
My son has a place in a local preschool a few minutes walk away, and attends for 8.5 hours a day. We have flexible timings for pickup and dropoff, but most of the kids have gone home by 4pm. We only pay £150/month for a full time preschool place, including food/nappies etc.
In our local council-run preschool there are three teachers for 15 kids maximum, which seems ok to me. In my experience Sweden often doesn't have official limits on things like childcare ratios, because they expect organisations to do the right thing. If there was only one teacher to 15 kids there would definitely be a scandal!
I would say that the Swedish system encourages people to send their children to preschool from around 18 months (you get 18 months paid parental leave per child). It is not possible to place your child at preschool before the age of 1, and most join at around 14-18 months old for a shorter day (around 6 hours), before going to full hours if the parents are both working.
It would be unusual for a kid to not join preschool before they go to school aged 6. However, it is definitely not compulsory! Many people choose to keep their children home, or to only send them part-time. If you don't have a place in a preschool, there are so-called "open preschools" which function like playgroups. So the parents stay with the children and play together. They can drop in as much or little as they want.
In terms of the societal expectation. Yes, you would expect a child who is 18 months or older to be in preschool. And there is some aspect of helping children to integrate into Swedish society through attendance in preschool (some might see that as indoctrination). But mainly it is seen a low-cost way for children to socialise and play and learn some independence, and to allow parents to work/study. I second the suggestion to listen to The Cold Swedish Winter, which looks at the indoctrination question in a humorous way.