What are usually considered the essential components of a smartphone?
The essential components of a smartphone typically include:

Processor (CPU)
Memory (RAM)
Storage (Flash Memory)
Display (Touchscreen)
Battery
Operating System (OS)
Mobile Network Connectivity (Cellular Modem)
Wi-Fi and Bluetooth Connectivity
Camera(s)
Sensors (such as accelerometer, gyroscope, compass, proximity sensor, etc.)
Audio components (such as speakers and microphone)
Of course, there may be other components or features that can vary between different smartphone models or brands, but the above list covers the basic components that are typically included in a smartphone.