how does online college work?

There are basically two styles of online classes offered by most universities: synchronous and asynchronous.

In a synchronous online class, the class "meets" at a regular time each week virtually, using a service like GoToMeeting, Blackboard Collaborate, or similar. It's basically like a Skype group call. The professor lectures, either standing in front of a camera or by sharing their screen and going through a Powerpoint presentation, and everyone listens to the lecture in real-time, much like a face-to-face class. In an asynchronous online class, the professor pre-records the lecture videos and posts them on the site the university uses as an online class portal, typically Blackboard, Canvas, or similar. The students watch the lectures individually, on their own time. In both cases, the professor posts assignments each week and it's up to the students to get them finished and turned in by the due day.