I disagree. All of them were used on different devices, with S60 being the most popular and used on most phones, just regular market differentiation.
For example: https://www.theregister.com/2011/01/12/symbian_history_part_...
(From my employers, but not by me and before I worked there.)
This whole series is well worth a read.
I disagree. All of them were used on different devices, with S60 being the most popular and used on most phones, just regular market differentiation.