The RFID reader module does all the radio interaction, activates the tag or card and reads data from it, storing it internally.
(eg. The IC on the RFID circuit board, in that article).
The MCU treats that IC or module as a peripheral, either polling it for any data or (depending on the type or RFID interface) just eg. receiving a burst of serial data when a tag is brought in to range of the reader.
So, that project does not involve you in any of the "radio" stuff, that's all pre-built; just power and logic connections between the RFID board and your MCU.
Note there are several different RFID standards, some operating on about 125KHz and some at 13.56MHz
125KHz are most commonly used just for ID, like door entry or garage door openers.
13.56 MHz ones, such as the "Mifare" standard, often have onboard data storage and can be written to as well as read from. And they have a unique ID number, so they can be used as simple door entry etc.
This is someone's project to do the RF side as well using an arduino and a few components; 125KHz is a low enough frequency for the MCU to do some measurements on the signal and decode data.
It pretty much has the function of the RFID module in the article you link to, sending the card ID number by serial data:
This RFID reader can read key fobs Tags and credit card size Tags also.
www.serasidis.gr