Newbie question: Does Wi-Fi Clone convert two networks onto one?

If I add the 8630 kit to my network (router with wifi connected devices in other room), what is the difference and/or additivity to the Wifi signal strength from the router and from the 8630 wifi extender?  That is, I add the adaptor and 8630 and then Wi-Fi Clone so that my router network name & password and the adaptor-8630 network name & password are the same. Do the connected devices see the wifi signal strength from the router increased by the addition of the extender signal strength?  Or do the wifi signals compete and/or interfere with each other? Or do the devices simply "pick" the stronger signal?

For example, router has 100mps download speed from ISP.  The download speeds seen by Wifi connected devices in other rooms (2G and 5G) are ~ 20 - 40 mps.  The extender is then plugged in to the same circuit-breaker circuit as the devices while the router and adaptor are on an adjacent circuit-breaker circuit - but all on the same household circuit & breaker box. What will the devices see now regarding download speed?