watch os

Hi, this is a follow-up to another question I had.

Hi, my friend has an iPhone 6s running iOS 15.4.1 and he wants an Apple Watch.

he is wondering if he should buy the Apple Watch Series 7 even though he heard that the upcoming iOS 16 is not compatible with his iPhone.

The reason why he is asking this is that he wants to get the latest watchOS updates but if you need iOS 16 to update the Apple Watch in the future he will not buy the watch

also,

if he doesn’t update the WatchOS in the future, can he still sync his activity, get calls on his Watch, send texts, play music, etc.

So in other words can the Apple Watch still be used normally

thanks