What we learn about the Borg is they either assimilate individuals of other races, or create their own young in incubation chambers.
As such, the Borg are a variety of races.
Why do they seem like one race?
Star Trek very much endorses the idea of "Planet of Hats": Each race gets a dominant characteristic, and they bang that drum really hard: ...
The Year of Hell episode opens by showing us Annorax, the Krenim officer and temporal scientist, removing an entire civilisation called the 'Zahl' from the timeline. Annorax and his crew have been in a temporal field for over 200 of their years - jumping through time making various adjustments to the timeline.
But what has this got to do with Kes? Well,