Editable mesh from blender into unity - how to apply terrain properly?

I am new to Unity. Have been using Blender for a few years in my free time and now trying to get into Unity because it seems like a better option in this case.

I am trying to export an unfinished concept mesh (from Blender) that shall be the terrain in the game when it is finished.
I want it to be handled with like a terrain in Unity (I just found out that this is possible but I have not really a clue about anything…) and be editable as well. Tried .obj but had some errors in the console. Took .fbx now after looking up the handbook.
The problem is still, first off, the mesh gets torn apart into four meshes because it got so many faces. Then, when I apply the components that are added when I add a normal “terrain” object, there is either no option to edit it or there will be added a white plane (I don’t want to use this because I already have a rough mesh I want to use) after I apply what the window asks me to do and I can only edit that part of the object but not the rough map conception.
Is there any trick to do as I am trying to, but in a different way? Is there any tutorial out there I could watch? I have been searching for videos for a while but did not find anything that was exactly dealing with the problem I am facing right now.

I’d be happy if anyone could help me with this. I am a complete noob with this program and I am just overwhelmed by the options I found so far. It’s completely different to the program I used before and I really have no idea how to deal with this without any helping hand.

Terrain is a dynamic mesh yours aint, u cant apply terrain modifiers to a mesh, in order to import your terrain which should be a square from an ortographic top view, you need to make height map out of it, to make a height map.
Height map is a grayscale map where absolute black means the deepest point, and absolute white the highest, unity uses 16bit win .raw files for height map.