The Raspberry Pi Foundation publishes the official Raspberry Pi 4 Model B schematic PDF on its product documentation site; search “Raspberry Pi 4 Model B schematic” on Raspberry Pi’s documentation pages to download the official files.
Before looking at transistors and resistors, start here. The block diagram shows the major ICs: Raspberry Pi 4 Model B Full Schematic
When designing a HAT for the Pi 4, you cannot just copy the Pi 3 schematic. The Pi 4 has different voltage tolerances. The Raspberry Pi Foundation publishes the official Raspberry
This page details the processor's "glue logic": Raspberry Pi 4 Model B Full Schematic
The schematic illustrates the LPDDR4 SDRAM integration.
Practical notes: