VIZIO D series TVs use the SmartCast platform, which offers a host of built-in apps and supports Google's Chromecast, Apple's AirPlay 2, and voice support for SIRI, Google Home, and Alexa.
I'm afraid you can't download other apps manually, but it does add new apps to the platform frequently. If you find the app you want to watch isn't on SmartCast, you can cast using Chromecast or Apple AirPlay on your phone or mobile device.