Over 200 bright quasars have now been discovered in the first billion years of the Universe, with this number set to explode again with the now-online Euclid space mission. Observations of the first quasars, serving as "back-lights", have revolutionised our understanding of large-scale-structure at early times. In particular, our picture of hydrogen reionisation has been completely re-written. This process, during which intergalactic hydrogen becomes ionised by the light of the first stars, has broken every expectation: it ends much later (z~5.3), is far more inhomogeneous, and is far clumpier on small scales than all models had predicted. In this talk, I will present these surprising results obtained in the last 3 year by the XQR-30 Large Program. Along the way, we will need to worry about the origins of the first supermassive black holes, and the growing mystery of their unchanging properties across cosmic time.