There are a couple ways to do this.
I have 2 Airport Extremes. One covers the lower basement area of our home and the 2nd covers the 1st floor and part of the yard.
I found there were 2 ways to do this. One is make them individual base stations (They relay the signal) and for me the preferable way is to connect them with an Ethernet cable.
The reason the Ethernet cable is best for me is that I do pass large graphic files between computers.
Some are on Wi-Fi and others are connected to a router.
The Ethernet cable does allow a faster connection at the secondary base statons.
See Apple's help page:
Dunc
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