This is a 50,000 feet view of how an AWS Application Load Balancer (ALB), an Auto-scaling group (ASG), a Target Group (TGP) and Route53 can work together.
ALBs can load balance your HTTP and HTTPS traffic forwarding them to registered targets in the target group evenly based on health check configuration specified in the target group.
The ASG provisions the desired number of instances into potentially multiple AZs (you control that when you create the ASG) to keep an even balance for HA and registers them with the TGP. Once the health checks pass for a target (instance) the ALB is ready to route requests to it.
The ALB itself can be provisioned in multiple AZs for HA (you should provision them in more than one AZ for HA). AWS creates a DNS name for it and creates its own HOSTED DOMAIN to store the related IPs. You can then create an Alias record in Route 53 pointing to this ALB name for DNS resolution for your users. Assuming you registered a domain called example.com and want to create a subdomain called acme.example.com you create an Alias record pointing to the ALB name in Route 53 for this subdomain name and that completes the loop.
Note that the public IP of the ALBs can change but Route 53 will keep it up to date in your example.com domain and return the latest IPs back to the users.
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