.1. It’s like The old pony express mail carriers. Each carrier can go a specific distance and then they give the message to the next carrier to carry for the next leg of the journey.
So any given repeater device only carries a message for about 40 feet from where it is. It won’t have anything to do with devices which are further away than that. At the end of that 40 feet it has to pass the message to another repeating device or to the hub.
So you need one repeater for Z wave about every 40 feet. And you need another repeater for Zigbee about every 40 feet.
If you are using z wave plus devices, they have a somewhat longer range, so you only need one about every 60 feet. Z Wave plus devices can repeat Messages for any other Z wave device, whether it is zwave plus or the older zwave classic. But they still can’t repeat for the zigbee.
The Samsung SmartThings branded devices are all zigbee. So a zwave device like the Fortrezz will never see any of their messages and cannot help repeat them.
.2. For your second question, yes, you are correct: Wi-Fi is a different protocol and cannot help your Z wave or Zigbee devices.
.3. A repeating device doesn’t boost the signal strength like a Wi-Fi booster does. It’s just able to pass along messages for other devices. But all the devices have about the same signal strength whether they are battery operated or mains powered. So you don’t need a second repeater close to the battery operated sensor. As long as the signal from the first repeater can reach the battery operated sensor, then that’s good enough.