Compatible to what apple watch

The problem is that all new Apple Watch models are shipping with watchOS 7, and will not pair to an iPhone 6, as the iPhone needs to be running iOS 14

That means the used market, and there are 2 issues.

1st the previous owner MUST have followed these instructions to allow a new owner to use the Apple Watch

https://support.apple.com/en-us/HT208855

If the previous owner has not done this, you can never pair to the Apple Watch, as the theft prevention features cannot be bypassed.

2nd, if the previous owner upgraded the used Apple Watch to watchOS 6 or 7, then you cannot pair it with your iPhone 6.

If you can overcome both of these limitations, then you can use an Apple Watch with your iPhone 6.

In reality, if you really want an Apple Watch, I would suggest planing on obtaining an iPhone 6s, SE, 7, 8, X, Xs, 11 or 12, all of which will upgrade to iOS 14 and pair with any Series 1,2,3,4,5,6 or SE Apple Watch.

Any unit running a watchOS version below 6.0.

(191804)