At usTwo I was one of three engineers who shipped the iOS app for Inflection AI's Pi. It took eight weeks.
Technically the interesting parts were server-side-event token streaming and custom text rendering. An assistant that streams its reply a token at a time is a different rendering problem from one that arrives all at once — the text is growing under you continuously, and it still has to lay out and scroll without flickering or jumping.
The UI is deliberately minimal. That was the product's whole posture, and restraint in an interface is harder to hold than it looks, particularly on a deadline.
Eight weeks is the number people react to, but the thing that actually made it hard was that the product direction changed daily. You're building against a moving specification, so the work is as much about what you don't harden as what you do.
It was three of us, so the credit is shared.