I guess you can't find it in the f-droid app, because it's hidden because of its anti-features. You have to enable in the settings of the f-droid client to show apps with anti features, than you will find it by search.
Edit: Also to enable opening links in the f-droid app you have to go to the app info in the android settings, where you can select the links you want to open with f-droid. Then your browser should redirect links to the f-droid clients. Here is a detailed tutorial about this, it's the same workflow with f-droid: https://gadgetstouse.com/blog/2022/01/28/open-facebook-instagram-twitter-links-in-app-not-browser/
That's weird. When I look up Libretube I can find it in my f-droid app. Says it needs Android 5.0 or higher, maybe your version is too old so you cannot see it?