AWS LB - How does being a regional service work?

See below from the ELB documentation which explains how when you enable multiple AZs for your ALB AWS will create nodes in each of those AZs. Often in diagrams for simplicity only one ELB is shown but in reality every AZ that you enable has the ELB node in it which makes it highly available.

Availability Zones and load balancer nodes

When you enable an Availability Zone for your load balancer, Elastic Load Balancing creates a load balancer node in the Availability Zone. If you register targets in an Availability Zone but do not enable the Availability Zone, these registered targets do not receive traffic. Your load balancer is most effective when you ensure that each enabled Availability Zone has at least one registered target.

We recommend enabling multiple Availability Zones for all load balancers. With an Application Load Balancer however, it is a requirement that you enable at least two or more Availability Zones. This configuration helps ensure that the load balancer can continue to route traffic. If one Availability Zone becomes unavailable or has no healthy targets, the load balancer can route traffic to the healthy targets in another Availability Zone.

After you disable an Availability Zone, the targets in that Availability Zone remain registered with the load balancer. However, even though they remain registered, the load balancer does not route traffic to them.