Basically the large soil volume gives you all the nutrients you need up front, the cover crop holds in moisture and fixes nitrogen into the soil and the bugs cycle the cover crop back into nutrients when you cut it and drop it. When you add nutrients, you basically just drop some compost near the root mass and water it in, and again the volume of the soil means that it's really difficult to overfeed. It's also easier to keep the soil moisture more stable with that much dirt, compared to daily or multi-daily watering in a 3 gallon fabric pot.

I highly recommend the Build-a-soil youtube channel just don't let him sell you too many potions because you can get kind of crazy with all that shit.