How do I view photo data on iMac?

Which metadata are you looking for?

You can see the the location, the title, and the date directly in the toolbar bar of the photos window. And the title or filename can be seen even below the thumbnails, also some badge overlays. You can enable this in the view menu -view > Metadata. set the checkmarks of the items you want to see. To be able to see the titles/filenames below the thumbnails you have to disable the tiled view with the square thumbnails. This can be done by clicking the button to the left of the scrollbar:

I am preferring this view anyway, only with the true aspect ration can I easily find the photos I am looking for. Otherwise it would be impossible to see at a glance if a photo is a pano or in portrait orientation and it is not possible to compare the composition.

You can also open the image file with Preview and use its Inspector feature to view the EXIF an IPTC metadata:

 In the Photos app, navigate to the photo you want to adjust. 2. Now, either swipe up on the photo or tap the info (i) button to view the photo's information.