YouTube has always been a little late to the design game at Google, but that doesn't mean the team isn't constantly testing little changes here and there. Apparently, the current design and location for the upload button is confusing to a large amount of mobile content creators, and so the company is testing a new design that places the upload functionality in the center of the bottom navigation bar.
Since the change adds an upload icon to the bottom bar, the notifications icon has been moved up to the top bar where the upload icon used to be. The subscriptions button will move to the right of the new upload for those in the experiment. The new design is in limited testing for Android users right now, but if the feedback is positive, YouTube says it will consider rolling it out more broadly.
Screens are (relatively) small on mobile devices, so it's important to make the most of the space. Uploading is arguably more important than viewing notifications, and this update puts the app more in line with the web version, which places the notification icon in the top right. I don't think I've ever willingly opened the notifications tab on my phone, so this seems like a change for the better.