The mouse double-click is a common way to select and open a file or folder in desktop operating systems. Interestingly we could do without it as it is possible to set up Windows to open a file or folder with a single click and other operating systems exist that don't use the double-click for example Puppy Linux. As it stands the double-click is popular and so it is the way we do things.
It would be nice if everything was done with a simple single-click but that is not the case. Instead all I can suggest is try a single-click and if nothing seems to happen maybe try a right-click or a double-click. There are no rules.