Where did this logic come from that all girls are the same and all boys are the same?

Painting whole groups with broad brushes is the typical way in which simpletons process the world around them.

Probably from someone who kept meeting the same person in different bodies.

I accept that boys tend to be more similar to each other than to girls, and vice versa...

But this is of course based upon a superficial examination of the corpi involved.

And statements comparing average, say, heights, strengths, etc., could be made but also shown to NOT hold for many specific instantiations of "boy" and "girl" humans.

It sounds like from someone who grew up very sheltered, either homeschooled or in a gender-separated school. Nobody sane who grew up in a normal class surrounded by boys and girls can believe that.

Have not a clue
But that’s most certainly not true

The same in what way