I’ve got an iPhone 6, whether Apple Watch 5 series will compatible with my phone?

No new, shipping from Apple, Series 3 nor Series 5 Apple Watch is compatible with the iPhone 6. All the new Apple Watches ship with watchOS 6, which in not compatible with the iPhone 6 iOS 12 version.

The safest thing is finding a new Series 4 from some store's inventory, as that should have watchOS 5.

If you choose to get a used Series 3, make sure you can always contact the owner and that the owner will correct any issues you run into. If you buy it online from a single seller, there is a chance they will disappear after they get your money, and if there are any issues, you will be out-of-luck.

The seller must follow these instructions

What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else

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

No, an apple watch 5 requires at least an iphone 6s or higher with ios 13 or higher.

depend on the watchOS, watchOS6 is not compatible with iphone6

if the watch 3 runs an older version yes, otherwise no