So maybe you using stadart unity terrain or any external software? “Pre-render” all terrain in your head before or building it in runtime? What problems you are faced while building your world?
If you want to quickly make something, Unity’s Terrain tool will work. However, it is a performance drag (especially on mobile) and lacks many features (such as overhanging or making holes in terrain). So if you’re doing a complex professional project, probably use an external terrain tool, but for most simple projects Unity’s Terrain will work OK.