This is something I was wondering as well.
Just as I was getting the hang of C9 I get asked to migrate over to Glitch, and even though I’m asked to create a project on Glitch, I see a sample project created on Heroku. It all feels quite inconsistent. I don’t have a good grasp of what the differences or similarities are between any of these sites.
I suppose it’s just as well we get a bit of experience with a few different platforms, though.