how to i download the installer for mac OS catalina???

how do i download the installer for mac OS catalina? or any for that matter. i am currently running high sierra. we can skip the part where u ask me if my computer is compatible and if i checked all the obvious common sense reasons as to why. yes i have also found the apple provided page that allows us to download older OS versions. when i click on one, it attempts to open an app... nothing happens. no apps actually open whatever the links actually are.

if anyone can help i'll be here for hours trying to get my life back together smh