Holy water is water, plus a specific prayer, and the prayer is what does the work, nothing about the water itself changes physically, but its treatment afterward changes considerably.
The blessing
Only an ordained priest, or a bishop, has the faculty to bless water for this use, historically through an elaborate rite of exorcism over both salt and water before combining them, a form still used in the Extraordinary Form and simplified in most modern rituals to a shorter blessing. Many parishes bless a large quantity at once, typically at the Easter Vigil, and use it to refill fonts and stoups throughout the year rather than blessing water fresh for each refill, so what sits in the entrance stoup may be considerably older than a visitor assumes.
Containers, and how it is disposed of
Holy water is kept in stone or metal fonts at church entrances for the congregation to bless themselves on entering, and sold or given away in small bottles at pilgrimage sites, the plastic bottles shaped like the Virgin Mary sold at Lourdes being a well known modern example of that trade. Because it remains blessed rather than becoming ordinary water again, holy water that has gone stagnant or is being replaced is not poured down an ordinary drain, it is returned to the earth directly or through a sacrarium, a special sink in some sacristies that drains straight into the ground rather than into the sewer system.