Hello Vasco, first of all congratulations for creating a food forest!! Good on ya!!! Secondly, thanks for asking the question on Permies, hopefully some more knowledgeable and experienced folk than myself will chime in as well.
I agree with you, you don't need whole layers of soil. Can you link me that video? I'd like to comment there.
A teaspoon of forest soil contains many yards of fungi and many,many, beneficial bacteria/archea, nematodes, protozoa and arthropods.
The idea being that you take the soil food web from one place and build it up somewhere else.
But if there is nothing for that soil food web to live in, it will just die. If it's too dry in summer because the trees are too small and not established, most species in the transplanted soil will be food for bacteria and nematodes and protozoa that can live in that environment. If the environment is right you will not need much of the original soil food web to multiply, it will go everywhere it can.
I totally agree with Deedee putting it in the planting hole will be sufficient.
The important fungi are the ones that bond with the trees rootsystem. They're species specific. So if you're planting locally growing trees in your food forest, go to the biggest healthiest and collect some soil from around the stem and transplant this in the hole where you plant your tree. Close the hole at the "mother"tree and come back a year after to repeat the procedure. You can dilute it in water and poor it around the plants you want infected with the specific soil food web.
For any tree conifer try to look for an abies alba which soils contain the most mycorrhizal fungi systems.
The way i did it, someone on Permies didn't agree and saw it as wasteful, unethical and dangerous to the tree. Imagine stripping whole layers of forest soil.
good luck Vasco, i hope you can keep us updated on how things are going.