ALB with HTTPs listener: Is there a full step-by-step anywhere?

Hi Stu. This guide may help you with the 'Step 0' piece. It covers HTTP listeners, but you can fill in the HTTPS pieces from the doc you linked.

https://aws.amazon.com/getting-started/hands-on/get-a-domain/

Do I need to first purchase a domain?

Strictly speaking you don't, although without your own domain you'll find the functionality and useability is limited. But if you just want to get familiar with load-balancing then you don't need one, no.

You can stand up an ALB, and behind there will be a target group inside which is (say) an EC2 instance running a webserver. The ALB will have a DNS name along the lines of [something].[region].elb.amazonaws.com and you can try to connect to that in a browser using HTTPS. But it will give you a certificate error, because obviously you don't own amazonaws.com so you can't generate & attach a cert for the domain.

Here you can click Advaned and then Accept (the exact words might be slightly different on your browser) and your browser will trust the [something].[region].elb.amazonaws.com site and the connection goes on to the target group and the EC2 instance within it.

It makes sense to have two listeners on the ALB, Listener #1 listening on port 80 (plain HTTP) and redirecting to port 443 (HTTPS) https://repost.aws/knowledge-center/elb-redirect-http-to-https-using-alb

Listener #2 listening on HTTPS port 443, and the ALB does the SSL offloading. Then pass through to the target group with plain HTTP on port 80. And the webserver (Apache, nginx, whatever) on the EC2 listening on port 80.

If you have your own domain, the Route 53 steps are laid out here https://docs.aws.amazon.com/Route53/latest/DeveloperGuide/routing-to-elb-load-balancer.html