In Earthly experience quarks are confined within small particles such as protons, neutrons, and other more exotic but short lived particles. Under more extreme conditions quarks can instead exist in a more unconfined form in larger "chunks" than just protons or neutrons. One example being a quark-gluon plasma which requires very high temperatures but is potentially denser than nuclear matter because it can be more highly compressed. Theoretically there might be other states of "quark matter" which might be stable in conditions other than those required to sustain a quark-gluon plasma. This could include "strange matter" and other exotic collections of large numbers of quarks. We've studied quark-gluon plasmas experimentally but we have never created other forms of quark matter so they still remain the subject of speculation and research.

It may be possible that there are routes to creating a higher density object than a neutron star that don't involve a quark-gluon plasma but we don't know for sure. In any event, when a neutron star is crushed down enough (whether via creation of a quark-gluon plasma or through some other form of matter) an event horizon can form. Very early on you'll have a bunch of very high density matter which is still being pushed inward with incredibly force/speed due to collapse and this will feed directly into the event horizon causing it to grow. At some point the rotation of the star/black hole creates a little bit of a delay in how fast matter can fall into the event horizon as an accretion disk forms and that matter takes several seconds to fall in. The black hole feeding on ultra-dense matter in its near proximity releases a lot of energy through the glowing of the accretion disk and the formation of axial jets (due to tightly wound magnetic fields from the accretion disk), creating some of the most energetic events in the universe: gamma ray bursts.

In any event, like everything else in the universe black holes typically have some amount of rotation, which causes the event horizon to have an ellipsoid shape. When an event horizon is created it's sort of like a space-time relic of the gravitational fields of its progenitor. As matter falls into the black hole it basically encapsulates the old event horizon in a new larger event horizon.