Current questions and answers regarding the map editor.

Is it possible, and if so, what is the best way to make it so that, for example, the entire map is a forest (Elven and Human), and there is a portal, and when a hero enters this portal, they are transported to other lands (let's say, completely demonic)? But please don't suggest using the Dungeon faction.

Hypothetically, you could create objects that mimic another type of terrain and cover the entire map with them. But that would be tedious, messy, and just awful. Otherwise, there's no other way.
Or, you could create a campaign – wouldn't that be a good way? Enter the portal => end up on another map (win there, but you can creatively change the victory message).