A reasonable solution is to get a free "address" from the post office with optionally phone notifications for mail
I really wish there would be more work to try to at least add some kind of alternative path here, given America's growing homeless population. Leaving things to the goodwill of family or friends seems to me like a dereliction of duty by the state.
Like for some deliveries you need to sign a receipt that will be legally binding, the post office would take the role of handling those.
I ended up finding some kinda sketchy-feeling services aimed at people RV living, and not much else. I wasn’t able to find an official solution to the problem of “I need to receive mail but have no address” (there may be one, but in solid 60-90 minutes of searching I didn’t find it, but did find a lot of people complaining about the problem)
But a lot of people might receive mail at a friends' address with permission. But, you still need to have a friend or family with a stable address who is willing to help.