The Phone 7 Plus originally ran iOS 10. It received five major updates and can be updated to the latest release of iOS 15 – currently 15.8.4.
You cannot update it to iOS 16 or higher. The current version of the YouTube app requires iOS 16.0 – so basically Google has abandoned support for your phone.
I see that the original title of your thread was "iphone 7 plus new." Apple came out with that phone in September 2016 and discontinued it (in the U.S.) in September 2019. If this is a second-hand phone, it may be "new" to you, but it would be far from a new phone. You may want to keep this in mind when purchasing a replacement phone. Part of the reason that older iPhones tend to be cheap is that they either have reached the end of their update life, or are much closer to it than the phones that Apple includes in their new lineup.