Firstly is there a very simple way to generate lower poly assets from your high poly quickly? or is it all remade by hand
secondly, do 3d object files have a place inside them that saves the different LOD’s or do you literately have to save out 3+ object files and then tell the engine that each one is displayed for each distance?
This is something you need to keep in mind before starting to model.Because LOD creation is best done by going low poly to High poly rather than opposite way because it won’t give best results.
Although you can convert high poly to low poly using Decimate modifier and selecting Un-subdivide.But Again that may not give an ideal result.So you might have to re-tweak low poly model.
But always try to go from low poly to High poly and as you go keep saving various “instances” of your model based on LOD
Secondly,
As far as I am concerned engines save different copies of each model (Not sure though)
Some games use different models for LOD(quake 3), then others use 3 different models compiled into a package/binary(source engine)