In the later half of last year, we saw the first hint that YouTube may be launching a new subscription tier or service called YouTube Plus. Aside from seeing that particular name spread to the other apps in the YouTube family, there hasn't been anything new about the 'Plus' branding specifically. However, it seems there's yet another name popping up: YouTube Premiere.
The features discussed below are probably not live yet, or may only be live for a small percentage of users. Unless stated otherwise, don't expect to see these features if you install the apk.
Three new images were added with the latest YouTube Music update. It's not unexpected to see the lower two, Music and Red, but the Premiere logo is brand new.
Unfortunately, that's where the clues end. There is no new text to suggest what the Premiere name will be used for. It's possible YouTube is launching yet another service or subscription tier to live side-by-side with Red, Plus (when it launches), and its TV service. It's also possible Premiere will replace the Plus name, though the string containing the YouTube Plus text hasn't been changed.
It's also a possibility that Premiere (or Plus) may not even be for the general public, but instead have some correlation to the content creators that upload videos to YouTube. At this point, there's not much to do besides speculate on what's to come.
The APK is signed by Google and upgrades your existing app. The cryptographic signature guarantees that the file is safe to install and was not tampered with in any way. Rather than wait for Google to push this download to your devices, which can take days, download and install it just like any other APK.