I guess I look at the natural cycle of the forest around me--stuff grows, then leaves and herby stuff dies back and falls to the ground
Including a continual "rain" of nutrient rich liches and bits of moss being blown out of the
trees, organisms and freezing cold break down cell walls and digest stuff. Rain leaches nutrients back down into the soil. Annual plant rots dies and rot down in the soil. Then there's the mycchorizal threads between trees and plants exchanging nutrients and doing something to them chemically to make them easier for the plants to digest. SO yes, humus or good dirt does indeed build up, or rather it works its way down, because you know how you need to replace soil in a
raised bed after a few years because plants are sucking mass out of the dirt and using it to grow mass of leaves and dirt--so soil doesn't build "up" so much as we think it does.
SO, if you were to dig down in a no tilled, mulched bed after a few years you'd see good tilth and dark goodness farther down into the soil than when you started.
I layer mulch throughout the year, to reflect the natural cycles. So say through summer I'm adding chopped grass/weeds as a mulch(which surprisingly is "gone" by fall), then through the winter I add leaves, clear spots to throw down kitchen scraps, sprinkle
ash over all when I clean out the stove, so it's like a continual mulching and composting right on the bed--just like in a natural environment. I try not to cover perennials(unless they are tender or sturdy like asparagus). If I prepare a new bed, I pile it high with slash(like 2 feet high) and in a year it's ready to plant. So I dont' have a separate compost pile where I make compost(which would lose nutrients to leaching unless it is kept covered).
Ash from
local trees is good because trees go down far into the sub soil and bring up minerals and nutrients, which they release either by being burned up or rotting. So if your
land is naturally wooded, but is cleared now and you're growing stuff on land that has been untreed for a long time, it's missing those deep nutrients brought up by trees.
For your friends question I would ask them, "what happens to a plant in the wild?" and see if they can answer that(...). How does the woods/prairie/desert get along without us applying hordes of compost(fertilizer, pesticides, herbicides...).
SO yeah, in setting up a garden you would most likely need the compost to get going(unless you have kickass dirt to begin with). But once it gets going, it's "chop n drop".