Features
QNAP NASes share a common operating system, whose latest version is QTS 4.3. You can read or download a comprehensive brochure here. QTS 4.3 continues QTS 4.2.2's look and feel with some minor visual tweaks. You still can't resize many of the windows, whose default width is more than 1024px. The screenshot below shows the "hamburger" menu and dashboard flyouts exposed.
QNAP QTS 4.3 desktop
Of course, some of the features found in the high performance NASes, such as real-time 4K video transcoding and virtualization won't be available on entry-level NASes such as the TS-431 series. But Plex is among the installable apps, all of which are shown below, along with all QNAP's "Station" apps. A noteable omission is antivirus, however. Complete specifications for the TS-431P can be found here.
QNAP QTS 4.3 desktop
QNAP has a YouTube channel that hosts the QNAP College. There are videos covering hardware installation, initialization, setting up myQNAPcloud, Network Access protection and a tutorial on how to backup your Mac. There is a total of 29 Videos that provide support on a wide range of topics.