hmm.. is the word "flat" confusing you? I don't think you actually understand the flat earth theory. I find many times the word "flat" confuses people and many don't know where to get the information on how to understand flat earth. 

The mountains are easily explained. Think of it like this. You have a table and you fill it with water. The you put the land in the middle, at the beginning it's just flat, but you add mountains, you put my soil in one spot and a mountain is creates. 

Did you play in a sandbox as a kid? You never made a mountain?

Just put the continents of the earth on a table and the mountains still exist. The land is not actually "flat"

They call it flat earth because the world we live in is on some sort of flat tabled.

Look in the picture below the land is not flat but the dome is on a flat table 

https://media.letsfo.com/images/2025/06/16/flat-earth-terrain.png