Is Apple Watch seven compatible with iPhone 6

Apple Watch Series 7 is not compatible with all the iPhones. Apple Watch requires iOS 13 or later. Even if it was compatible you would miss out on tons of feature. Since your iPhone 6 is already old consider upgrading to a newer model. If a brand new device is out of your budget you can always go for refurbished iPhones. 

An apple watch 7 requires an iphone running at least ios15 or higher,your iphone 6 will only update to ios12 maximum.

No. Get a new phone, your current one is obsolete (released in 2014, 32 bit processor, 1gb RAM).