Short answers, pulled from the story.
A stellar black hole, or stellar-mass black hole, is a black hole formed by the gravitational collapse of a star. They have masses ranging from about 5 to several tens of solar masses and can be the remnants of supernova explosions, though other formation mechanisms may also operate.
Stellar black holes range from about 5 to several tens of solar masses. Until 2016 the largest known stellar black hole was 15.65 solar masses, and a rotating black hole of 62 solar masses was detected forming in a merger in September 2015. The smallest known sits in 2MASS J05215658+4359220 at 3.3 solar masses.
By the no-hair theorem, a black hole can have only three fundamental properties: mass, electric charge, and angular momentum. The angular momentum of a stellar black hole comes from the conservation of angular momentum of the star or objects that produced it.
Stellar black holes in close binary systems become observable when matter transfers from a companion star and heats to several hundred million degrees, radiating in X-rays. The black hole is seen in X-rays while the companion is observed with optical telescopes, and masses are derived by combining X-ray and optical data.
Some models predict that black holes cannot form directly from a single collapsing star in two ranges, the lower and upper mass gaps, roughly 2 to 5 and 50 to 150 solar masses. The upper gap arises from pair-instability supernovae, which blow apart stars of around 130 to 250 solar masses without leaving a remnant.
When a collapsing star exceeds the Tolman-Oppenheimer-Volkoff limit it forms a black hole rather than a neutron star. All identified neutron stars have masses below 3.0 solar masses, so compact systems above 3.0 solar masses that lack neutron star properties are most likely black holes.
The Milky Way contains several stellar-mass black hole candidates closer than the supermassive black hole at the galactic center, mostly members of X-ray binary systems. Examples include Cygnus X-1 at 21.2 solar masses, GRS 1915+105 at about 14 solar masses, and V404 Cygni at about 12 solar masses.