Q: How are Aperture Projects handled in Photos?

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

LN

Photos does not support projects.  The basic library items are moments. The moments are created automatically, based on the capture dates and locations. You cannot control how moments are split or move images between moments.  There are no folders to group related moments. This has been replaced by collections and years.

The only folders you can create are folders for albums and smart albums. Your projects will be replaced by albums, grouped into folders with the same names as in Aperture.

You will see differences for other library items as well - the smart albums may not show the same photos as before, because Photos does not support all constraints, your books may look different because of different templates.  You keywords will be replaced by a flat structure, and images with brushed adjustments will migrate like externally edited, with a new quasi master.

I posted a user tip with a few notes on the migration:  Notes on Migrating an Aperture Library from Aperture to Photos for Mac

You're welcome.   Did you read the document Larry pointed to as well?

Good Luck for the migration. Make sure you upgrade your Aperture 3 to Aperture 3.6 while it is still available, I'm not sure, if it will be possible, once El Capitan will be released at the end of the month.

See:  Updating to Aperture 3.6 After the Release of Photos