Has anyone got the covid app and been told to self isolate?

My friend has just been told he's got to self isolate for 5 more days.

But looking online it said you should isolate for 10 days from the exposure to that person but he onky got the notification today so now I'm worried as he did drop my kids presents off and we saw eachother at the door.

He said he litreally had only been to tescos and another supermarket of the supposed close contact with someone who's tested positive

Like hows this app work?