Go to the Pokemon Centre in Eterna City and speak to the brown haired girl, she will hand you the Pokétch app "Friendship Checker".
Find poke app 8(berry searcher) by a girl on route 208.
In the app store
You can get it from Professor Oak, in Pal Park.
the poke radar isn't an app it is a key item, i think you get from prof rowan after completing the national dex
at the poke app store in jubilife city when you talk to the president on the right door.
You can find the app in the Pokemon Center in Eterna City. Talk to one of the girls.
You get a special poke tech app and It shows you. The one were the Pokemon look like they are floating, you just touch them with your stialis.
You get most of them after you defeat the elite four. Hi Jake Nichoals
go talk to the poketch app guy in jubilife city after getting your 3rd gym badge
get the Poke tech App Poke happyness
Talk to the lady in the Eterna Pokemon center, the one near the stairs. She will give you the app. Not sure if it works for DP
In Pokemon Diamond the Friendship Checker (also called the Happiness Checker) app is #6.