1. Upgrade my ios device ?

From what's been published about newest [past beta] iOS.

..A finished iOS grade won't work on older iPhones such as

an older 6s, etc; ones like you have. ~Not on mine either.

The latest iOS only works on iPhone 8 and above.

No, iPhone 8 and above will receive iOS 16.

Anything less than an iPhone 8 will will stay at iOS 15.X.X

Your iPhone 6S cannot install beta 16