A microcontroller packs a CPU, flash for code, a small amount of RAM and peripherals such as timers, GPIO, ADCs and serial buses onto one chip. It boots in milliseconds, runs firmware directly or under a small RTOS, and can idle in deep sleep drawing microamps. That combination makes it the default brain of battery-powered IoT devices, from door sensors to asset trackers.
The trade-off is capability. Memory is measured in kilobytes to a few megabytes, so an MCU cannot run Linux, a full TLS stack can be a squeeze, and complex analytics have to be reduced to what fits, which is the territory of TinyML. When a product needs a camera pipeline or a rich user interface, designers move to a microprocessor. The IoT device types and constraints guide maps this choice.
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