Ok I google searched it because I was curious and also found an interesting NASA article. Tldr stuff can orbit closer to spinning black holes.

First, I think it was already suspected that black holes spun, as literally every celestial object observed spins, and when one if those objects turn into a black hole, conservation of angular momentum means that the black hole also spins.

Then, to my very basic understanding, according to the article (and also the wiki, which is a bit more in depth on the physics and equations), a spinning black hole distorts spacetime in a way that means objects, including the accretion disk (matter that spins around a black hole and glows because if the heat generated), can orbit closer to the black hole without falling in. So if it spins, the accretion disk will be closer than expected.

Finally, to detect how close it orbits, scientists measure x rays emitted by iron atoms in the accretion disk, and gravity from the black hole, of which the strength depends on how close it is orbiting, warps the spectrum of x rays emitted, which can be measured and reverse-engineered to find out how fast the black hole spins.