They probably use L2TP with IPsec to get Layer 2 transit. Doing that over Wireguard would require Gretep or something similar
They are using BGP and routing nodes (backbones), recreating a mini IP (layer 3) network I think.
I've used raw wireguard in a p2p fashion to interconnect LANs. I run wireguard on each segment directly inside the network routers.
Just make sure all LANs are using a different subnet. A /24 is standard. Then configure all the peers and you get a fully peer to peer network. No relays. You only need one side of every peer "pair" to be reachable from the internet.
I do have a small management script to help peer discovery (dynamic IPs) and key exchange, but it's not strictly required. With a dozen nodes or so, it's maintainable manually. Wireguard supports roaming natively, as long as one peer can reach the other.
Very little overhead. ICMP, TCP and UDP support.
The only use case I can imagine is a legacy game which performs a server search by broadcasting/scanning the local network. And even then - most of the time these games had server browsers.