Can my iPhone 6 Plus update to IOS 15 because it’s still says iOS 12

No, 12.5.5 is the latest update a 6 plus can update to. It doesn't have the hardware needed to run anything newer.

Go to Settings > General > Software Update.

Update your iPhone, iPad, or iPod touch -

https://support.apple.com/en-us/HT204204

No, it can update to 12.5.5, an iPhone 6 plus does not have the hardware to run iOS 15.

Can my iPhone 11 didn’t update to IOS 15

For iOS 15, you will need an iPhone 6s, or newer.

and Sanju0047, you can totally update your iPhone 11. Head to settings, press general, press software update.