I want iOS 13 for my iPhone 6

Can’t get to iOS 13 any more.

Of what can be reached, iOS 15? And iOS 15 on iPhone 6? Not going to happen.

iPhone 6 has half the memory of the oldest supported iPhone, among other differences.

Everything prior to iPhone 8 goes unsupported later this year with iOS 16, too.

That is not possible, as it lacks the hardware to update beyond iOS 12.5.5.