Why would the Youtube app not work but Youtube in the browser works fine?

Because the functions available in the app are controlled by the developer of the app. The Internet, of necessity, must be available to
as many browsers as possible
or the site is limiting their
traffic.
The same thing used to
happen with the old Flutter
app. It had limitations built in
so using the mobile site was
always preferable.

I don’t us any app for which there is a website available because, to me, it just wastes space on my phone or tablet which could be better used for apps which can’t be accessed by a browser.

The limitations imposed by the apps are usually too annoying to deal with for me. I’d rather used the website. So it takes a few seconds longer for pages to load? Not a big enough problem for me to justify using an app which cripples functionality. I’m not in that big a rush.

You might need to update the app.
I’ve had that happen and it was because I hadn’t updated it.