Change software from OS X El Capitan to Catalina

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

How to prepare the Photo Libraries before upgrading to Catalina - Apple Community ➙ read first  if you haven't already upgraded to Catalina

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

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

How to upgrade to macOS Catalina - Apple Support

Do keep in mind that 32 bit apps no longer work in Catalina. So if you are running older software, chances are some of them may not work.