Where can i rate pictures in Photos?

There are no custom star ratings in Photos, only the "Favourite" heart.

All star ratings from iPhoto will have been migrated to Photos as keywords - "5 star", "4 star", etc.

You can continue to use these keywords. If you drag them to the "Quick group" in the Keywords Manager, you can use the same keyboard shortcuts "5", "4",... as before to assign your ratings.

Launch the Keywords Manager by entering ⌘K.

Bring up the Keyword pane (Command+K), select the photo(s) you want to rate and apple the appropriate keyword to it. To be able to use the one key short cut drag the keywords up into the Quick Group area.

You can scroll thru the photos with the arrow keys on the keyboard.