That's what Hamachi does. They're still called VPNs.

You have no idea unless you try it. It is different. for example this network device is meant to make a network between all your iot devices, or over a cross provider cloud infrastructure. It does not route all tradfic through a vpn server. Nodes connect directly to each other. you do have a fallback openvpn mode for iphone devices though. You cannot really imagine if you havent tried it (and that is a problem for zerotier anyways)

Have you tried Hamachi? That's exactly how it works. It's not a VPN that routes all your traffic, it's a virtual LAN. You get a private IP, and can communicate with other devices on the same network over their private IPs. And yes, nodes connect directly to each other.