As Greg says - it's mostly proprietary.
Every vehicle manufacturer will have procedures which stipulate, to some level or another, the steps used in packaging, even down to socket, spanner and hand clearances, although, from what I see these days, some companies don't give a monkey's cuss about how serviceable their products are.
Stuff like torque reactions, engine rock, transmission wid-up, heat clearances etc. will figure in it all.
Solid modelling is today's de facto method of packaging, but there's still a lot to be said for full-size mock-ups at some stage of the design process to see whether an average body can get in/out and whether the engine can actually be installed without hitting every other component under the hood.
Bill