How can I access all my photos in IPhotos if I upgraded to MacOS Catalina?

Did you read my post, above? Specifically the second paragraph where I say:

Hold down the option (or alt) key and launch Photos. From the resulting menu select 'Choose Library' and point at your iPhoto Library. It will do the rest.

iPhoto was discontinued 5 years ago. It is no longer developed or sold. It was replaced by the more powerful (and free) Photos.app that comes with the OS. You can migrate to that app very simply:

Hold down the option (or alt) key and launch Photos. From the resulting menu select 'Choose Library' and point at your iPhoto Library. It will do the rest.

If you prefer not to use Photos there are several 3rd party apps that you can use. Which to choose depends on the kind of photographer you are and your budget.

Another method to migrate from iPhoto to Photos is to drag the iPhoto library onto the Photos icon in the Dock or in the Applications folder.

"hovering" is not involved. Launch the new Photos app - the one that looks like a multicolored flower - with the option key held down.