You already have your one site Galera cluster set up - see my previous guide on how to do so.
Your site 2 is connected to site 1 using a site-to-site VPN (such as by using WireGuard with OPNsense). This tutorial does not cover the setup and troubleshooting of site-to-site VPN communication.
You have set up a load balancer on site 2 (similar to what we did on site 1) such as by using HAProxy on OPNsense.
A site 3 VPS (you can set up a cheap $5 USD/month VPS, or use a free one) that will act as an Arbiter.
Recommended: set up a web server VM (or more) on site 2 to ensure that traffic that comes to your site 2 firewall is distributed to your site 2 local web VM(s) and the local nodes of your galera cluster (that we are yet to deploy) for lower latency.