opening building door with iphone instead of rfid card

Potentially, but not natively with an iPhone.

The potential options would be:

• An external RFID reader that you could connect to your iPhone either by Bluetooth or by USB-C. In theory, these readers can communicate with your iPhone, allowing you to use it to scan RFID cards and open access doors.
• Third-Party Apps: Check the Apple App Store to see there are any apps that can turn your iPhone into a RFID reader. If there are, they most likely will require additional hardware, such as an accessory that plugs into your iPhone 15's USB-C port, to function properly. What would be important here is that this reader be compatible with the RFID system used for door control at your school/univerity.
• Integration with Access Control Systems: This is most likely what you are looking for. Some access control systems offer mobile credentials that can be stored and used on your iPhone. These systems typically require coordination between the access control provider and your iPhone's software. Once set up, you can use your iPhone to securely access doors just like you would with a traditional RFID card.

As you can see, there is likely no simple solution without coordinating this with your administration, as they would be the ones to work with the folks that provide the RFID door access controls.