So how come it's a ring of plasma that forms and not a sphere? Like we can see the hole unobstructed? Is it because gravity tends to clump things together? I guess Saturn's rings are the same?
Saturns ring, the flattening of the accretion disk, the fact that planet systems will have most planets more or less in one plane and that spiral galaxies are flat (much thinner than they are wide at least) all have the same reason, yes. Basically most orbits align due to initial orbital momentum and the stuff that orbits in other directions sooner or later experiences enough friction to be forced into the common orbital plane.
Conservation of Angular momentum collapses shapely sort of things into a disk