In the overworld not really. I don’t think you can even place blocks below the lowest y level.
In the nether you can use an Ender Pearl to get on top of the bedrock “roof” in the nether and then create a hole to easily get above or below. It’s completely flat up there and mobs don’t spawn on bedrock, so it’s a great place to build farms or just travel easily.
Do you do anything then? Can you build on the underside?
In the overworld not really. I don’t think you can even place blocks below the lowest y level.
In the nether you can use an Ender Pearl to get on top of the bedrock “roof” in the nether and then create a hole to easily get above or below. It’s completely flat up there and mobs don’t spawn on bedrock, so it’s a great place to build farms or just travel easily.
That sounds cool and fun
This only works in Minecraft Java, not in bedrock. Bedrock doesn’t allow building on the nether roof