From PSU: “The Folding Screen Monkeys are the fourth boss in Sekiro: Shadows Die Twice. They are found at the to of the Senpou Temple, Mt. Kongo area. You can reach them by interacting with the bell on the table, in front of the Main Hall Sculpters Idol.”
Source: N4G PC Sekiro: Shadows Die Twice Folding Screen Monkeys Boss Guide Senpou Temple, Mt. Kong