I had a bat in my room once. I left the window open. It flew around the light (which was turned off) and then flew out again. It was cool.
Flys are everywhere, if you leave your door open it'll have come in through there. Why did you have to ask?
(Though tbh in my up-upstairs rooms at home (converted attic) there's always dead flies near the closed windows. I swear they sneak in through the floorboards
)