Gamesharing question

If I want to share with my nephew in another state, Do I just make his PS4 my primary and my PS4 his primary? I can't wrap my head around how this works. Who would have to buy DLC if we both wanted to use it?

Yes

PS4 #1 @ your nephew's house

-Listed as your primary on your PSN account.

-All users on this device can play all of your digital content on their account.

PS4 #2 @ your house

-Listed as primary on your nephew's account

-All users on this device can play all of your nephew's digital content on their account.