He mentioned what he did in the video. Pretty sure he went on ChatGPT and asked it for some lyrics in the style of Eminem and on Future Rave.

Copied & pasted the lyrics in an AI voice generator and probably had to use some Eminem acapellas to train it.

There's plenty of AI Voice generators I watched a segment on one a few days ago https://youtu.be/OEoU9k5hApM?t=1061