Selenium is the oldest of the three big tools, and it does the least for you. That sounds like bad news. It is actually why Selenium questions come up so often in interviews: nothing is hidden, so your answers show whether you really understand testing.
Here is the model. Your test is a program (here, Node.js) that talks to a driver. The driver controls a real browser. Every command, like find this element or click it, is a message sent to the driver.