you shall not ask, you shall buy, therefore it is gooped - at least you get promised a relative input noise of -131,3 dB.... you don't need to know, how this dream-value (theoretical limit: -130,5dB) is achieved (probably with liquid helium) - simply believe it, buy it and tell it to your friends.
IIRC the XTX131 was designed by Andy Gemmell-Smith and was sold as a balanced to single ended mic amp with low noise performance and high CMRR for about £30 per unit
I've never seen a schematic but it might be similar to the Alice Soundtech mixer front ends (for which I may have a schematic) which are std diff amps with feedback to the bases of a second pair of transistors