what happens to iphoto when i upgrade to catalina?

It no longer runs.

iPhoto was discontinued more than 4 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.

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 

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

He already told you, use the option key when launching photos to convert your library.

I am not sure what you mean about images "linked" to movies and DVD's - iDVD won't run either - but you can keep your iPhoto Library. When you do the conversion, Photos creates a new library from your iPhoto Media. Your iPhoto Library will be unchanged.

See Old Toad's links at the end of this thread:

https://discussions.apple.com/thread/250966952

I'm not sure what you mean by 'what can I expect'?