How do I transfer iPhoto to my updated Photo on Catalina?

Try to open the iPhoto Library in Photos by double clicking the iPhoto Library. YOu should be asked, if you want to open the library in Photos.

Alternately hold down the options key⌥ while launching the Photos.app. Keep holding down ⌥ until you are seeing the library chooser dialog. Then select your iPhoto Library from the list of photo libraries.

When Photos opens, you should be seeing all photos and albums as described here:

How Photos handles content and metadata from iPhoto and Aperture - Apple Support

Here are some user tips from Photos for Mac  User Tips that will help you in your transition from iPhoto to Photos:

Notes on Migrating an iPhoto Library from iPhoto to Photos for Mac

Moments in Photos are the New Events.

External Editors in Photos Are Here

Photo Editing Extensions for Photos for Mac

Post 9/30/2018 Printing Options for Photos - Apple Community

Ordering Books and Calendars on a Mac that cannot be updated past El Capitan 

Use Project Extensions to order print products in Photos after September 30, 2018 - Apple Support

Create and order print products in Photos with Project Extensions - Apple Support

Great 😊