Well, it closes the apps somewhat arbitrarily, but they definitely arent all open. If the most recent app is a resource-intensive game or something it will more aggressively close things. but you can have more open if they are all lightweight apps.

But the switcher shows every app ever opened and tries to pretend these apps aren't being closed. Right now for instance, i just checked and as best i can tell, the last 3 were open. going back any further and i could tell the app had to reload. but i can scroll to the left back forever. the one on the furthest left i easily haven't opened for months

I do this also - but wonder because I'll have some app open, phone gets warm. Close the app and it cools down.