Some versions of the UniFi firmware/software have a "zero handoff" feature which (among other things) places all APs on the same channel.

You don't have to use that feature in which case channel assignment is done as in any other AP.

I just use 1 and 6 for WiFi and make sure Sonos is set for 11 unless I can setup a totally wired Sonos system and get channel 11 back.