Money is money. Samsung also makes a bunch of stuff for Apple, and they don't seem to think twice about it.

I'm not clear what Samsung makes for Apple (someone might know, but not me), but it makes sense to share the more complex parts. One team does all the expensive design work, and then both benefit because the design cost is over more devices, plus more hardware means better scale factors in manufacturing.

The above is on a case by case basis. So they may share some complex part, and some other (possibly more complex) they decide isn't worth sharing.

The main thing Samsung provides Apple is their iPhone displays, but they’ve also supplied chips in the past (I’m not sure about now).