Hi Black Finch,
I bought the original models from the Unity Asset Store and then hired artists to make heavy modifications. Tools used varied by contractor, but mainly Blender and Maya.
I then use my custom mesh combiner to assemble the characters with hair, body proportions, clothing, etc. The mesh combiner also rigs the skeleton accordingly.
I would recommend making your own character models instead of buying assets if you have the ability. It makes for a more unique looking game and is a fraction of the cost/effort.I’ll never do it this way again :)
Kind regards,
iLEWD