Possibility to use >=2 Networks with k8s, MicroVM or FullVM

[fl0wm0ti0n]

I wonder how I can build up an infrastructure where i can split networks up like for example following picture:

            public
               |
network1 --- FW-VM --- network2
     | vm1               | vm2
     | vm2               | vm4

As far as I know I need the possibility to add more than one network to a vm or container if I want that.
But by now via Terraform I can add just one network and via playground I cant even add a custom network.

As far as I know im not able to route from one network to another network i deployed since they are separated. if that would possible it would be a security problem. for example if a user deploys a network and i do so too, it should ever possible that we can reach us. even if i deploys 2 network for different customers of mine...

There is a wide range of use cases where you can need more than one network in one infrastructure

So if i want to separate 2 local networks and want for example that one network can reach the other one but not the other direction, i see no possibility right now.

I mean I can build up something similar, where they can reach each other via public:

  public ----------------- public
     |                       |
network1                 network2
     | vm1                   | vm2
     | vm2                   | vm4

but, wouldn't it not be way more efficient locally?

I know if I containerize I should be able to build up virtual networks, I know in k8s and docker it works! Is this also the case on the grid?