Q: sort photos by date modified

You can't.  The only date sorting is by date created or date added and those are in different parts of the library.

Tell Apple what additional features you want in Photos via https://www.apple.com/feedback/photos.html

Also Photos doesn't have the feature to edit in an external editor (ie Photoshop)

Photos now relies on plugings for interacgting with 3rd party apps like image editors or watermarking apps.  We'll have to wait for the 3rd party developers to provide us with the plugins.  So tell Adobe that we're waiting.

Please explain how to "The only date sorting is by date created or date added and those are in different parts of the library."

Thank you

In Photos view the only available sort is by Photo date - in All Photos view it is by date added

LN