Actually no...things are a bit different.
First of all, all buildings and units are created when map time is 0.00 so it doen't take more time on loadscreen.
About terrain i'm not sure..they might actually make it also when game starts.About their modes..that's pretty simple too.
For example, -rd or random draft create lots of heroes and only the attempt to load so many heroes would provoke huge lag, so they load them as buildings and when you choose them then they load the units.
About hero abilities or skills, these are imported as .slk files.