I’ll let someone else speak to how Hummingbird is built (I think the actual build pipelines are 100% deterministic, but IMBW). But for classic Fedora, you can in fact see how Fedora is built. It’s all open source. You can’t shell into the actual build systems unless you’re in the infrastructure team, but that’s all.
The best entry point for understanding how Fedora images (except CoreOS) are built is https://forge.fedoraproject.org/releng/pungi-fedora . That’s the repo that contains Fedora-specific configuration for the Pungi tool that runs Fedora composes. Mostly what Pungi does is create jobs in Koji.
There’s a shell script in the pungi-fedora repo which is run every day (by this cron job) and runs Pungi to create the day’s Rawhide compose. Those composes wind up here; there’s a symlink for the most recent one. In each compose there is a global log file which logs Pungi’s overall execution and from where you can track most of the individual tasks it kicks off. You can track from there to the individual Koji jobs. There are also various other log files you can poke at, I won’t try and explain them all right now.
The individual image build jobs in Koji mostly use Kiwi these days. Fedora’s Kiwi job templates are here. Some still use Lorax; the configuration for that is kinda spread around the stack. Koji’s source is here.
Basically all of this stuff is open, you just have to know/figure out where to look. 
You can, if you figure it all out, stand up your own Koji and duplicate the entire compose process. It’s somewhat easier to replicate individual image builds, especially with Kiwi.