I want to make a flat world in survival, so I need villages, but I'm not sure if they spawn in Flat. By the way, I'm on Bedrock edition.
No, flat worlds don't generate villages. It's only a flat world filled with grass up to
y = 3 with Bedrock in the bottom. You can't really customize your flat worlds just like in Java, like adding villages or changing the floor.
In Bedrock, your default game mode will be Creative when you set your world to be flat, but turning the gamemode Survival will put the world into Infinite instead.
So there's no way of getting a survival flat world with villages in Bedrock Edition.