A lot of people seem to think of Khan Academy as a bunch of videos. Many have also seen the exercises and articles. Those things are all pretty static (though it gets more complex when you consider how much content there is and how many languages it's localized into).
There's a whole bunch of dynamic behavior around that static content. Keeping track of progress to tell a learner how they're doing, plus to help recommend the next place to go in the content. Reporting on progress to parents and teachers. Letting teachers create assignments and manage their classrooms. Bubbling up information to school districts.
Content pages have discussions and clarifications. There are notifications to tell students about new assignments, for example.
There are connections to tests, like the SAT prep or integration with the MAP test, which involve connecting our accounts with external accounts in order to help students based on those test results.
And a bunch of other stuff that isn't coming to mind right now because I'm just naming things off the top of my head.
Doing all these things across a user base of millions of monthly users can get quite involved.