Gallery shortcode and attaching images to posts

I would like to add a simple gallery to my post.
Apparently Gallery Shortcode it says how to do this.

The [gallery] shortcode makes no mention of where it pulls the images from, only that if images are attached to the post, they are pulled from there.

I attached images to the post and they were inserted directly into the html for the post. No gallery.

And for the Post format on the right side of the edit post screen there is Standard, Aside and Gallery. Is this related? When I click Gallery it has no effect.

So how does this actually work? I’m lost. Thanks-