Update iOS 17.0 is required immediately to download you tube and other apps

If you have services that require that your iPhone runs iOS 17, and your iPhone 6 Plus cannot run this version, you will have to upgrade to a newer iPhone.

To run iOS 17, you will need to get an iPhone Xr or later model.

Ref: iPhone models compatible with iOS 17 - Apple Support

Youtube does not require iOS 17. But it does require owning an iPhone which can at least run iOS 14. And since you own a nearly 9 year old iPhone 6, your phone is not capable of running iOS 14 or anything higher than iOS 12.5.7. So, if you want Youtube and other apps, you're going to have to retire your very old iPhone and buy a newer, more modern iPhone as your only option.