The soil used depends on what you want to grow in the bed. If the bed is deep and the plants are not expected to have deep roots, then a filler material in the bottom third will work fine. Note, branches, leaves, etc., will compress over time and the bed may need additional soil a few months or years later.  Using purchased bags of soil may be cost prohibitive depending on the size of the bed. Using soil from neighboring ground may depend on its condition. A soil test from the U of MN is a good idea if you are not using purchased sterile soil.
https://soiltest.cfans.umn.edu/