While we weren't able to find anything else particularly interesting (note: Music Key is not linked to this update, previous versions had it, too), YouTube 5.18 does include some interesting strings referring to upload trimming. Specifically, these strings:
<string name="upload_edit_video_tutorial_title">Trim your video</string>
<string name="upload_edit_video_tutorial_description">Drag the handles to trim.</string>
<string name="upload_edit_video_tutorial_zoom_description">Touch and hold to get a closer view.</string>
<string name="upload_edit_video_tutorial_got_it">Got it</string>
It looks like a new upload interface is on the way for YouTube's Android app, and it's probably about time - the current UI is pretty dated. Trimming will certainly be a welcome tool on the go, because honestly, who shoots a movie on their phone and starts and ends it exactly where they'd like? Unfortunately, the feature does not appear to be active just yet.
Regardless, you can get the new 5.18 APK on APK Mirror, below.
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.
Version: 5.18.5 (51805300)