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 practical walkthrough of the keypad-reading code via the Keypad.h library: makeKeymap, row/column pin arrays, and the getKey function.
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.
An explanation of ASK modulation in 433MHz RF modules, and wiring them to Arduino via the RadioHead ASK library.