What are the typical components found on a modern smartphone's main board?

A system-on-chip (SoC) processor plus RAM, flash storage, power-management chips, and a cluster of wireless and sensor modules.

Modern phones squeeze nearly the whole computer onto a few chips: the SoC fuses CPU, GPU, and modems on one die, and the RAM is often stacked directly on top of the processor (package-on-package) to save board space.

Typical components (a Samsung Galaxy S20 Ultra teardown as example):

• Processor: Qualcomm Snapdragon 865 (8 cores) with integrated GPU
• Memory: 12 GB Samsung LPDDR5 RAM, overlaid on the processor
• Storage: 128 GB Samsung flash storage
• Power: Maxim power-management IC, Qualcomm power-amplification modules
• Wireless: Qualcomm 5G modem, Skyworks RF module, Qorvo WiFi module
• Sensors / radios: on the newer S25 Ultra teardown a separate RF board adds an NFC controller, ultra-wideband transceiver, an STMicroelectronics 6-axis accelerometer & gyroscope, a Bosch pressure sensor, and an AKM magnetometer

Go deeper:

• System on a chip (Wikipedia) — how a phone's SoC bundles CPU, GPU, memory controllers, caches, modem/RF and camera blocks onto one die, with real examples (Snapdragon, Exynos).