I can't pair series 9 watch with iPhone 7 due to low IOS

That’s correct. It’s not compatible. “Apple Watch Ultra 2, Apple Watch Series 9, and Apple Watch SE require an iPhone Xs or later with iOS 17 or later.“ Buy Apple Watch Series 9 - Apple

It isn't possible to pair those.

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It does. I literally copied the quoted part from the link I shared with you, which is the page you buy the Watch from. It has an entire FAQ section for you.

At some point all hardware just cannot handle newer software. 🤷‍♀️

Neil is right… Your basically screwed now.