Korean folk religion is populated rather than governed. There are a great many spirits, they are attached to places, households, functions and histories, and no one of them presides over the rest in the way a sovereign deity presides elsewhere.
The mountain spirit, honoured in small halls at Buddhist temples across the country, is among the most widespread. The seven stars, the household guardians, the spirits of the gate and the kitchen and the site, and the dragon king of the waters all hold recognised places. The arrangement is functional and local: what matters is which spirit has jurisdiction over the trouble at hand.
The pantheon absorbs. Buddhist figures, Daoist immortals and Confucian moral categories all appear inside it without displacing what was there, and the mountain spirit halls attached to Buddhist temples are the clearest architectural expression of that comfort with overlap.
The most distinctive feature is how readily historical people enter it. Generals, loyal ministers, wronged officials and princes who died badly are enshrined and addressed, and the tradition continues to add them. A death that was violent, unjust or unresolved is precisely the kind that produces a spirit with power.
The practitioners who deal with all of this are the mudang, who conduct the gut, the ritual in which spirits are hosted, addressed, entertained and asked. The office is entered by call or by inheritance, depending on the region.