I have a network 192.168.88.0/24.
I've established a VPN tunnel to a network 192.168.0.0/24 via L2TP.
Router/Devices on 192.168.88.0/24 are able to ping to the other side.
For beginners learning sake, I want to translate 192.168.200.0/24 as a 1:1 mapping to the IPs on 192.168.0.0/24 via the VPN via the 192.168.88.0/24 network.
So, if I ever encounter another 192.168.0.0/24 network connected via VPN I could then use 192.168.201.0/24 etc etc.
Would somebody be kind enough to explain how NAT works in this case along with some firewall/NAT rules. I don't quite understand how the router picks up the packet, adjusts it, then moves it onto the VPN interface. A wonderful mystery!

Thanks
Chris