Ubiquiti started in outdoor wireless, point to point and point to multipoint. All their outdoor radios, cameras, and unifi APs are all running a version of openwrt.

EdgeRouters started on vayatta and then that went away (AT&T acquired Vayatta) and then that forked into VyOS.

Some of the Ubiquiti EdgeSwitches are running some other OS, but I can't remember what it was.

Source - Spent 14 years helping alpha / beta test, debug code and wireless problems, etc.