How do I open iphoto on my mac pro ?

If you have OS 10.15 Catalina, then, simply, you don't. 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.

If you have more than one Photos library just launch Photos with the Option key held down and select the library of your choice in the window that appear:

If you want to merge the two libraries into one you'll need the paid version of PowerPhotos and follow this PowerPhotos tutorials:

6. Copying between Photos libraries

Copying albums and moments

Copying individual photos

Importing new photos into a library

Merging libraries - *

Previewing your copying

Handling duplicates when copying

Copying limitations

Copying and merging with iCloud Photos