Explaining how RFID works and the MFRC522 reader module, which runs on 3.3V logic over SPI, plus reading a card's ID from Arduino.
A general explanation of the Micro SD card reader module: the SPI protocol, voltage conversion, and wiring it to Arduino via the standard SD library.
Wiring a Nokia 5110 display via the SPI protocol to Arduino, with a technical note on signal voltage protection resistors.
A comparison of UART, I2C, and SPI protocols by wire count, speed, and number of supported devices, with a guide to picking the right one for your project.