The interference pattern only appears after firing many many photons. They’re still a photon (packet of energy) when creating an interference pattern. It’s the distribution and probability of its location that changes. Not the physical “shape”.

Edit: to clarify further

When not being detected, it’s still just one dot that appears on the sheet. As more and more are fired, the interference pattern shows to show as each photon hits the screen

It’s all about the distribution possibilities.