So, how in the world do people clean this much build up?

 

Aerosol engine degreaser and a pressure washer should do it.

 

Use SuperClean in a foaming spray-bottle, and a power-washer. Mine was so filthy, I had to power-wash it twice, and scrub it by hand once.
Looks beautiful now...

 

For starters, take it to the local pocket-full-of-quarters car wash and leave the biggest mess there.
But let the engine cool for some time first. If you shoot cold water at hot spark plugs, they shatter.
Castrol Super Clean works great. Purple power is the same thing. Cheapest at WalMart.
Wear rubber gloves. The stuff is caustic. It will eat the finger prints off of your hands.

 

That looks like undercoating (like a Rust Jones type product) with dirt on top, more than just pure dirt.

Is there an area that you have tried to clean yet?

WD40 makes a good cleaner. I've used it on a rag to clean up some caked on old grease and wax. It was fairly mild to the paint. There do seem to be varying degrees of clear coat over the color coat in the engine compartments of our cars. Some look as shiny as the outside, others (like mine) have a more matte finish (meaning less clear). As a result, some areas come up with a very modest amount of Bright Red on the rags I used.

 

I use the ZEP Purple Industrial cleaner and that stuff will burn the crap out of your hands but man does it eat through caked grease and dirt!

 

You going to run the risk of oxidizing the paint, and even more so any bare aluminum (water pump, heads, intake manifold, carb etc will turn chaulky white) if you use degreaser by itself as its very caustic. A Decon shampoo like OG Decon, or mix some APC with a good quality shampoo is next best. Use an IK foam sprayer dwell for 5 min and use a boards head brush to agitate. 40deg nozzel on the pressure washer to rinse. Plastics and rubber I use Megs #40 or Griots.

 

I know on some of the detail forums there are popular choices in Meguiars Hyper Dressing, 303, Carpro Pearl, etc. After a good cleaning spray on and come back and wipe off as desired. Then upkeep to avoid having to give a deep cleaning again. I kinda like that, spray and walk away. I'm lazy, lol.