VDD is 3.3V, you can see on the schematics and I guess the serial communication pins are also 3.3V. There are FL- and FL+ which are powered on by a separate output on the AC transformer and there's also an output from transformer called Vdisp, it has +37V and it goes to VDDH on the display, if I'm not mistaken, through some transistors.
Actually I think it does because there's nothing happening on the bus when there are no changes on the display. I had to frantically press the remote control to get it to capture anything. Also, as my receiver is broken, I have a floating message on the screen all the time with a long sentence indicating an error and while this screen is active, I'm unable to capture anything so I assume that there has to be some kind of controler inside which is moving the letters.
I can experiment with it but I will have to tear down the whole amplifier to get this screen working independently so ideally I wanted to know in advance because I'm currently building a new digital board for this amplifier with ESP32, ADAU1452 DSP and Raspberry Pi Compute Module 4 in Kicad so I wanted to confirm everything before I finish the board, and when I have the board, I will be able to test by using everything as it is.
If I won't find an answer here, I will probably tear it down completely, and try to power-up the front board that contains the display and then experiement with Arduino. The good news is that the whole power supply for the display is on that same board (all outputs from the transformer for FL and Vdisp are going directly to the board).