Noob question : How would you explain in the simplest form the difference between FOSS and source-available. In other words, what does Remotion do not have that would make it FOSS?
While you can look at Remotion's code on Github (that's what we mean by source-available), the project has a custom license that doesn't grant you the permission to do anything you want with it. Remotion retains ownership of the code (i.e. you cannot simply fork it and give it away for free), and when you are a company with more than three employees, you have to pay Remotion if you want to use their code to render videos
Remotion has a custom license that requires you to have a Company license if you don't meet certain criterias for the free license.
Revideo just has the MIT license.