How can I access all of my photos in iPhoto? What do I have to do to bring them into the new Catalina OS?

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.

Open the Photos app. Select its Preferences... > General, and choose your old iPhoto Library.

John: that just selects the system photo library. It has no role in migrating from iPhoto.