Since MPTCP is just a regular client-server (point-to-point) Layer 4 protocol that extends normal TCP (on a per-socket basis using the sockopt IP_PROTO_MPTCP), you need to build an entire ecosystem around it, kind of like with WireGuard. On top of that, you have to add an extra interface layer with IP-over-MPTCP or Ethernet-over-MPTCP to make transparent L2/L3 traffic possible. How this is actually supposed to work isn’t standardized at all, but there are some experimental solutions for Linux.