How to set up routing between two networks?

Is that how the network is physically wired? I suspect not. I would expect you to have a wifi network, or one where the nodes link to a central hub via CAT-5 cables. Or you are using a network of virtual machines.

In any case you need the nodes to understand how to cross the subnet boundary. This is normally the job of a router. Often a reasonably priced intelligent hub or broadband modem can do this by defining multiple subnets.

If it needs to be done on one node (top right on your diagram - That is helpful, well done. Extra points for explaining the config!) then the problem can be restated as "how do I have two subnets on one interface". And that has already been answered here

There will be minor changes because you will be using netplan, but you will get the idea.