Standard Interface Between LCD and GPU
iPhones have used MIPI-DSI (MIPI interface).  This is designed for low power (very low voltage swing and low power modes).  It also has a version for cameras MIPI-CSI.

There are other similar interfaces around as mentioned by other posters.

These modern interfaces for mobile devices typically use very low voltage swing (+/- 200mV for MIPI).  Run at high speed >1Gbps per lane and only have a few signal lines.  They are designed to be able to refresh an LCD screen at 60Hz or more and millions of 24-bit pixels.  Low power is an important aspect to provide long battery life.

The complex display driver is usually on the LCD/OLED glass itself or there would be many thousands of connections required.