My fancy Ubiquiti access point has an RF scan tool. Here’s what it found in my house in San Francisco, Noe Valley.
Not a great visualization; the yellow bar fills up the trapezoid. Mouseover tells me the 2.4GHz channels are about 20-30% “utilized” with “interference” of -95 dBm (noise floor?). 5.0GHz channels are mostly 0% utilized except for 112 and 136, which are about 20%. Interference there is -91 dBm for most, -96 for a couple.
My iPhone sitting nearby can see about 4 neighbor’s WiFi networks. Also probably a lot of 2.4GHz cordless phones, including mine. In practice 2.4GHz barely works through the house and 5.0GHz is terribly unreliable, probably because the walls are lath and plaster faraday cages.