I am working on a post-apocalyptic world with relatively advanced
small-scale militaries. In what ways are landfills useful as sources
of rare earth metals, glass, or other materials?
Edit: Mostly looking for rare earth elements to recycle and use in
electronics. Wondering if it is feasible and at all easy.
Maybe, but we have to take into account where the landfill is and when it was in operation.
In recent decades the United States and many other countries have limited what items can go into a landfill. Likewise, there are other places with few, if any, restrictions to landfill contents.
Even in places that did put restrictions in place, they didn't go back to old landfills and dig out all the undesirable junk, they just didn't allow it in the new landfills. So, in the US, landfills opened after the mid-'90s would have little in the way of electronics. But the older landfills would. There are thousands of such places in the US. We buried a LOT of stuff up until the '80s! There was a dump of Atari games in the '80s and maybe there is more out there.
Changing the dump mindset took years of effort and people willing to dig up the past in order to disprove landfill myths.
If it's key to the story, there is a good possibility you can find some history to back up your ideas.