UART
and it stands for universal asynchronous receiver/transmitter, it is one of the fundamental/basic ways of communicating the serial to another information source wirelessly. UART is a block of circuitry (hardware device) in a microcontroller that works as a link between your serial (computer) and parallel forms or an integrated circuit. it translates the data (income) to the serial. and it has 2 wires RX (receiver) and TX ( transmitter) to transmit and receive serial data.
after receiving the data from the data bus, the transmitter (TX) adds the start bit, parity bit and the stop bits to the data frame that is limited by a maximum of 9 bits. and then the receiver (RX) discard the start bit, parity bit and the stop bits and converts the serial data back into parallel and transfers it to the data bus on the receiving end.
as mentioned it’s a microcontroller so you will find it inside the modem on your laptop. or your raspberry pi. arduino. GPS shield and Bluetooth models.
Useful resources: http://www.circuitbasics.com/basics-uart-communication/ , https://www.edn.com/electronics-blogs/embedded-basics/4440395/USART-vs-UART–Know-the-difference
SPI
http://www.circuitbasics.com/basics-of-the-spi-communication-protocol
I2C
http://www.circuitbasics.com/basics-of-the-i2c-communication-protocol