Isn't there even an open implementation of the network/coordination layer (Headscale)? But regardless, the best term I've heard for these types of networks is

Isn't there even an open implementation of the network/coordination layer (Headscale)? But regardless, the best term I've heard for these types of networks is overlays. You can add "zero trust" or whatever to make it seem more fashionable, or VPN if you're old school. I remember using Hamachi for this type of use back in maybe the early aughts.