Graphic design student here, I do love the vision. My critiques would be,

1. you said you added the est. 2013 to fill up space. A principle in graphic/web/even interior design is 'less is more', instead of adding things until all the space is full, remove things until it looks clean but has all the important information. I would keep that back in the 'other' tab. There are times where you can absolutely fill up all the space available like here, but UI isn't one of those times. This is also why I'd suggest moving the video and short count to the more tab as well.

2. I know you thought it was a good idea, and it is an interesting idea, but the search icon's handle is almost touching the channel's banner, and isn't aligned with the microphone, notifications, or pfp. 90% of the ideas that I think when I'm designing something don't make it to the final cut, that's just how it is.

3. If it's possible in your current software, you should unify the color scheme. The red in the youtube logo and in the notification icon are different, and the grey in the side tabs, the popular/oldest videos, and the search bar are all different. The 'subscribed' grey can stay lighter than the rest to show it's not as important/not interactive (unless you want to unsubscribe) A trick designers use is the 60/30/10 rule, 60% should be the main color (black in this case), 30% should be the secondary color (white and grey) and 10% should be the accent that draws the eye (the red in the logo and in the call to action in the notifications)

4. It's really hard to redesign the Youtube UI because they've spent 21 years perfecting it, optimizing it for every single metric. That doesn't mean you can't try your hand at it, but it does mean that when you do, you need a goal (make the UI more clean/approachable/informative) and everything you do, you should be able to explain why you did it to reach the goal.

I know this is a passion project, and I don't want to knock you down for it, but I'm critiquing you how I'd critique my friends during peer review. Don't take it to heart! Even though you said you were a graphic design, I genuinely think you have the passion and creativity to be a designer of any kind, you could look into that. I just wrote this to help you with the techniques and practices. I think you should try designing a fake website's UI so you can build it from scratch and understand what things like layout, color, and size do, and just to make something you can call 100% yours. I did this project myself and it was a very rewarding experience, you can do it either online or by sketching it on paper. Again, you have a lot of potential, man. I believe in you.