How to set up a relay/network extension?

Installing and Using OpenWrt Network and Wireless Configuration
1

End Goal
All I want OpenWRT to do is to create an open network that then runs traffic through a closed network. An implementation similar to most VPN routers.

Question: Does and if so how OpenWRT allow this.
Specifically, how does one add the network for networking.

Sorry if this has been created somewhere before.

Hardware:
Raspberry Pi B 3+ (no adaptors)