My computer says my ride to work is 9.1 miles ,Endomondo says 8.5.
Does Endomondo take into account up and down or just assume the world is flat?

endomondo pings every few sec (approx 10) so cuts corners where your comp reads every waver/wobble

depends on how accurate the bike computer is calibrated too
my computer (VDO something) is typically about 1km more than any form of GPS app after about 25km.
GPS distance is pretty good usually (except lots of fast downhill switchbacks). GPS altitude can be anything from acceptably accurate to a totally made up number, even from the same data in 2 different apps.

Now I’ve checked it ,Endomondo says my ride to work is either 8.5,8.7 or 8.9 miles on exactly the same route.