SDN vs pfsense and or opensense

The process for setting up pfsense or opensense in proxmox and then messing with it looks something like this...

first install opnsense, add multiple network adapters before logging in. one of those adapters will be setup using just a default bridge to the existing lan network and one of the bridges will use the same interface but will have a vlan tag on the interface. this way all traffic is created inside this vlan.

the next step is to install an ubuntu desktop system on the proxmox host as well and setup 1 network interface there without vlan tag untill after ubuntu is installed. then add the vlan tag that matches the interface on the pfsense vms lan interface. this way youll be able to get access to the web console.

When trying to figure all this stuff out I was unclear on how connections should work between machines and software defined networks.