Q: Re photo vs iphoto.

Albums and folder are identical in iPhoto and Photos - and neither iPhoto nor Photos uses folders for photos - folders only are used for albums or other folders - photos are only in albums

this describes how things migrate from iPhoto to Photos - How Photos handles content and metadata from iPhoto and Aperture - Apple Support

LN

I had organized my photos in folders in iphoto.

You mean albums, right? Folders in iPhoto can only hold albums and other folders, not individual photos.

That being said when an iPhoto library is migrated to all albums will be retained and all iPhoto Events will be converted to albums.  This user tip describes what happens: Moments in Photos are the New Events.

These user tips from Photos for Mac  User Tips will help you in your transition from iPhoto to Photos:

Photos 1.5 vs iPhoto 9.6.1:  Features and Capabilities

Moments in Photos are the New Events

External Editors in Photos Are Here

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

Photo Editing Extensions for Photos for Mac