We use cloudflare. If you look up the IP address for your public domain name, you will get the cloudflare IP, yes. If you lookup the IP address of the CNAME target, you will get the heroku IP.
If you just use something like https://poof.io, then it would be @. Depends on your site.
There should be a few historical IP addresses, but you would create an A record for each of them.