This depends on the receiver. The G4 SlimPWRpatch for the LG G4, for example, is attached to the back cover of your phone. To see a full instruction video for the G4 SlimPWRpatch, click here:
iQi mobile for iPhone is attached directly to the lightning port of your iPhone. It's held in place with a small sticker and a soft case or cover.
The rest of the receiver cards do not necessarily need to be literally stuck on your phone. The card itself is held in place by the back cover of the phone. However, there is a small sticker on the cards, in case you want to make sure the receiver stays in place.