my iPhone 6 plus wont charge nor power on how to power it on

See this Apple article: If your iPhone or iPod touch won't charge

https://support.apple.com/en-us/HT201569

iPhone 6 Plus OR iPhone 6s Plus ?

FYI: iPhone 6S Plus is 6 years old, having the battery replaced by Apple or an Apple Authorized Service Providers may resolve the issue.

Have an iPhone 6S here, in normal use it's fine, but it got really hot only during the last update to iOS 15.

Have the battery replaced by an Apple Authorized Service Providers (about 6 weeks ago), because now it is still possible.

For older iPhone type is a bit trickier, I don't see any older iPhone type in the list, see here:

https://support.apple.com/iphone/repair/service/battery-power

If you are going to replace the battery, by Apple or an Apple Authorized Service Providers read this Apple article:

Get your iPhone, iPad, or iPod touch ready for service

https://support.apple.com/en-us/HT201557