Compared to the Arduino Uno of old, modern microcontrollers are absolutely tiny — especially for the amount of processing power and I/O you get. But if you need something really small, like ...

Raspberry Pi Pico's built-in bootloader allows you to write firmware via USB or SWD. However, in some situations, other interfaces may be useful. Picoboot3 now offers the options of UART, I2C and SPI.