The best set up is two mains-powered devices every 40 feet of the same protocol and on the same network as the sensors you are trying to reach. (If all devices are zwave plus, you can make it every 50 to 60 feet).
Zwave allows for a maximum of 4 hops from the hub, though, so if that’s not enough, you’ll need to use zigbee. Shorter hops, but you’re allowed 15 of them.
The devices for repeaters are usually pocket sockets, plug-in sensors, light switches, in wall relays, or in some cases lightbulbs.
Battery operated devices do not repeat.
However, a lot of people don’t want to spend the money for two repeaters in each zone, so they just go with one. There’s nothing wrong with that, it’s just that with two the network tends to be more effucient because every device has at least two choices.
The hub itself counts as a repeater for both zwave and Zigbee.
Personally, I like to test range using pocket sockets because they’re so easy to move around. And you can often get them for 20 or $30. Then after I figure out where I need repeaters (which can vary depending on architectural issues like water pipes in the walls, cement, drywall, even some kinds of wallpaper) I can then decide if I want to use wire devices like wall switches or relays.
But it just depends on your specific layout. If you know you’re going to want a smart switch in a particular place, you might as well install it and let it be one of the repeaters for that protocol.