Your home feed can default to "All," which shows content from all communities across all federated instances (not just Kbin and Lemmy) or it can default to Subscribed, which shows content only from communities you're subscribed to. Lemmy has a third option called "Local" which shows content from all local communities. Turning off federation for yourself in Kbin is similar to the local option in Lemmy, but allows you to still switch between All and Subscribed (ie, all local or subscribed local). Lemmy doesn't currently offer a "local subscribed" view.
The hamburger menu next to your name allows you to toggle between Subscribed and All. In the dropdown menu when you hover over your name, under Settings, you can select your default view (Subscribed vs All).
My personal preference is to default to subscribed and see content from across the Fediverse that I choose to see. This way I'm not missing out on any communities or magazines on federated instances that interest me. I sometimes switch to All with the Commented sort to see what the current zeitgeist is (where are the biggest conversations happening). I don't see much need to de-federate my view entirely since I can block communities/magazines, or entire domains, which means even under All I'm not bothered by content that I find annoying, offensive, etc.