Connect 2 routers using a vpn tunnel

[edoardottt]

Hi everyone!
Sorry, but I'm a newbie and I'm trying to learn new things here on OpnSense.
This is my network topology (the image topology.png, if you can't see it: https://postimg.cc/FfspGcf5) and I would like to connect these 2 routers (running opnsense) using a vpn tunnel and not the usual tcp/udp unencrypted traffic on the link connecting them.
How I can achieve this? Do I need a site to site openvpn tunnel? Since I have two LANs behind each firewalls I don't know if this is the correct choice (?), or maybe a host to host?
I would like to just route the traffic to one router to the other, so for example the subnet 100.100.6.0/24 should be able to reach the others two behind the other router using the tunnel.
I started using openvpn but without big results...

Thank you in advance,

Edoardo

[errored out]

The example you are giving will not work.  Not to sound harsh, but it would seem you may need to learn basic networking.  Your giving route-able addresses on an internal link/network.  To make matters worse, you are double N.A.T.ing is some areas. Then there is using a VPN internally, which many would suggestion you should not do.

If you give an explanation  what you are trying to accomplish (aside from setting up an internal V.P.N., given your main objective) and why you are using 2 routers, you most likely will receive better suggestions or the "correct" method of application. 

First, in many cases 1 router/F.W. is sufficient. There are reason to use 2 F.W.'s, as complex configurations / external requirements (sharing Internet connectivity between 2 separate entities) would need.

Second, You need to use a non-routable address for local network traffic.