Take a look at your Settings, Update & Security, Windows Update, View Update History Link then search for the Definition Updates section which is nowhere near to the top of the page, and expand it.
Here's mine from this machine:
As of this writing, Windows Defender definition updates are being handled via the Windows Update mechanism and clearly are done twice daily. It would only make sense that Windows Defender itself would be doing cyclic checks to see if the date-time stamp, or version number, of the definitions it has loaded are older than those stored wherever the updated ones are stored, and loading the newer ones when they become available.
Windows Defender also has a cloud component. It's become a far more powerful security tool, using many additional features it never previously had. It is meant to be something that needs no care and tending and my experience with it indicates that description is accurate.
I do believe, though, that there is a move in the works to move Windows Defender definition updating back into the core Windows Defender Security Center functionality. To me this makes far more sense. Security suites, anyone's security suites, should be fully self-contained within the suite. All updating, status of updating, etc., related to the suite should be handled by the suite and checked via the suite. I will not guarantee, though, that I'm not creating the previous out of thin air. There are just so many things I can keep track of, and this is definitely something that is at the bottom of my, "important to keep perfectly straight and at the front of my mind" category.