I'm sorry for a long post but I wanted to explain my workflow so you can correct me if I'm doing something wrong
that's no problem. This is why we are here.
First of all, there is a difference between the absolute height and the relative height. Subgrades, ai paths, reference lines, railtracks and some more using the absolute height. If you are working on a real map, you can probably can get the correct height from Google Earth or other programs. The height of the DEM terrain is not that precise, so I can't recommend using only the height of the DEM.
Now, should this subgrade be in width of road (+/- some meters) or should I shape the terrain of that area?
In that regard, I understand that OMSI-like terrain editing isn't yet implemented so should I just wait for that option or I should play around with subgrades?
Just play around with subgrades. When you get into it, it's a very nice feature in my opinion.
When that is done, then I can start placing splines there. So I use curbstones and set them to -0.15 and that will modify terrain further for road. Now, for asphalt I can use either lane splines or polygons and they need to be on -0.15 too.
This is correct, but I can't recommend the use of lane spline for asphalt. Mostly the polygons fit better in there.
So my question is do AI paths follow terrain or polygons/splines?
Everything is following the terrain, polygons and splines too, if you're using the relative height. This button right here toggles the height. When pressed you will use the relative height. Sometimes, e.g. when using subgrades, it is important to use the absolute height.