If you want to RUN a VPN, master iptables and enterprise networking: https://www.amazon.com/Linux-Firewalls-3rd-Steve-Suehring/dp... .
If you want to WRITE a VPN, read the source of a few openly available relatively simple VPN/tunnel implementations.
https://github.com/yarrick/iodine https://github.com/jpillora/chisel
Familiarise yourself with packet layers. Fantastic resource: https://github.com/netspooky/protocols/tree/main/broadcast_b...