Understand the traffic between WAN and LAN

I am trying to understand the underlying traffic flow in an OpenWRT router (or any router in general). And I find myself confused with the WAN - LAN traffic. Apologize in advance if this is a dumb question.

My understanding is that if we want to link any of the subnets, so that traffic can flow between them, we need to connect them via a bridge interface. Just like we need to build a lan interface to join the vlan of eth0.1 and wlan1. However, I am not able to find anywhere in the system that we joined the wan and lan. So I wonder what's going on here. I do see we can do very flexible traffic control between wan and lan with firewall. So is that the CPU would by default automatically connects together all the vlans that have a port to it? And the firewall here is only to add restrictions to these connection, rather than allowing it? (In other words, if the firewall is disabled, those vlans can then freely connect to each other via CPU with out limitation?)

Firewall basically allows or denies traffic.
If some interfaces are members of a bridge, then they all belong to the same broadcast domain.
If the interfaces are independent, then traffic is routed among them.

So when firewall is disabled, can the traffic from lan freely reach wan, or is the traffic completely shutoff?

Usually when you say "disabled firewall" it means that there are no rules and policy is allow.