Original: Certain Mennonite churches have a high number of people who were formerly from Amish congregations. Although more Amish immigrated to America in the 19th century than during the 18th century, most of today's Amish descend from 18th century immigrants.
Simplified: The ancestors of the Amish came from Germany in the 18th century, and now most of the Amish live in Pennsylvania (Lancaster County) and Ohio, in the United States.