I have a normal Elegoo TFT 2.8" display shield that also had an SD reader. I am trying to use it as a Nextion display. I want to do this because the Nextion tool allows you to make very nice looking sophisticated graphics and I wanted to do the same. Is there any possibility? Maybe some interpreter library?
No, you can't.
Nextion displays contain their own microcontroller with their own closed-source firmware on them. You need that firmware to have a clue what is in Nextion files, and only Nextion have that firmware.
While it may be possible to reverse engineer Nextion display data and write your own software to interpret it, it would be a huge job with little to no real-world benefit.
You cannot use a 2.8" TFT as a Nextion Display. Two are seperate things
Nextion Display is an HMI. You can configure prebuilt menus and graphics without coding.
The 2.8" TFT is a generic display module. It prints data according to the commands it receives. The user has to prepare the data to be printed. It can read data from an sd card, but the user program has to issue the commands for it.
Nextion displays use a generic display like the 2.8" TFT, an onboard controller with custom firmware running on it.