I am the type that likes to know the in's and out's of why to do things, or how things work.

My question is about soil compaction and soil improvement,
and the application of amendments (lava/green sand, compost, etc)
and how they get into the soil.

I have read that the first 7" of soil is the most important.
That is where the microbes and worms live etc.

When you apply amendments to the top of the soil, how do they eventually get down into the existing soil to do their magic of opening up the black gumbo that we have here in Dallas ?
I'm trying to fully understand the relationship of applying amendments and how the soil will actually be if you took a sample and disected it.
Are the amendments still going to be on top, or work their way into the soil, or just become part of the first 7" of the soil after applying amendments over, and over, and over?

Any info would be appreciated. I can't seem to find this type of info anywhere.

Earthworms and other beneficial soil organisms like sowbugs, maggots, beetles, etc., will do the most work far as mixing and putting nutrients and organic matter further down into the topsoil. Over time, 12-24" could be effected by natural forces.

Also keep in kind that powerful liquid biostimulants like aerated compost or seaweed tea or dry molasses mixed in stuff like fish emulsion, etc., will pull nutrients and beneficial microbes down further into the topsoil over time too.

Since I'm been doing no-till gardening for the last 7 years, I totally depend on the earthworms and microbes to mix and blend and pull the nutrients and organic matter further into my topsoil every year. The more compost and liquid biostimulants you use, the greater the potential to get deeper and richer topsoil over time.

Happy Gardening!

You can't get better advice than the Captain just gave but I will add another twist. Just like compost breaks down faster when you turn, water and aerate on a regular basis, soil will improve immediately if you till all the amendments into a new bed or garden plot.
That doesn't mean it's a better way, just different.
I also recommend you get one of Malcolm Beck's books. You will get all your questions answered about how nature works in the soil.
Tony