You're right, there is something called a "scrollbar", but many people don't see it because they are using macOS which for some reason hides the scrollbars by default.
Also phones/touchpads don't see scrollbars I think.
macOS does indeed show the scrollbar at all times when a input device other than a touchpad is attached.
My desktop Mac never hides the scrollbar, as it uses a normal mouse, however when I was using a Magic Trackpad the scrollbar would indeed hide unless I was actively scrolling.
Also phones/touchpads don't see scrollbars I think.