In some of the more traditional denominations—certainly Catholicism and Anglicanism, to name two—churches or church buildings are named after "saints". This is in contrast to the more evangelical denominations, where churches are generally either named after their local area ("Mytown Baptist Church" / "Mytown Methodist Church" etc.) or more free-form.
How do the traditional churches determine which saints to dedicate each congregation/building to? Is it the building or the church itself (the people) that take on the name?