I’m definitely not a Flat Earther, but I watched a documentary about them and a lot of them seem to think the Earth is flat and the sky is like a glass dome, and the other stars and planets are presumably just projected onto the dome somehow? Kind of like this: metro.co.uk/wp-content/…/flat-earth-1.jpg
As for what’s beyond the dome or why nobody’s ever encountered the edge of it, I honestly couldn’t tell you lol.