Typical house with a basement, there are about 4-5 renters.
There is a Cat6 line going from my main router to downstairs, which is then connected to a signal booster to give them all wifi.
I’d like to keep the renters completely seperated from my main house home network.
How do I seperate the signal booster/renters from seeing any devices on my main house router?
Should I connect a switch or router in between my main router and the signal booster?
Easiest fix is to replace the WiFi extender in the basement with another router. The tenants would connect to that one (whether physically or via WiFi) and the network provided by that router will be mostly separate from your own. If none of them are tech-y this should work fine but otherwise still allows them to connect into your network if they know what they’re doing and make an effort to do so. Think of it like a cheap lock—keeps honest people honest but someone with sufficient motivation can still break in.
There are more secure ways but they would involve replacing pretty much all of your networking gear with more capable equipment as you will need VLANs and a competent firewall.