This might be a good start
https://docs.theforeman.org/master/Configuring_Load_Balancer/index-foreman.html
The smart proxy is basically a content only host, which can also run Ansible roles and Puppet actions etc. You will find a good documentation inside of the Foreman manual.
Coming from Spacewalk this is a quite different architecture, you need to learn about content views, lifecycle environments and smart proxies. Many things are quite identical, from repositories to activation keys etc.
Unfortunately, I haven’t found a really good guide on how to compare Spacewalk and Foreman/Katello or even how to do a guided migration. All I can say is Foreman/Katello is the way to go.