Four hours north of New York City, a small team labors over monkish leather slip-ons with an ardent cult following. One ex-Birkenstock devotee explains their peculiar appeal.

"I was just scrolling through my For You page on TikTok one day, and a video came up," says Bottos, who lives in Etobicoke, Ont. On screen, a girl was decorating the pages of a notebook using ...