I asked an Episcopalian bishop last weekend how the patron saint for a new church is picked. He said that the people who are starting the church sit around and pick whatever saint they like, and that’s all there is to it.
When I was around 10 or 11, we started going to a new Catholic church. I remember parishoners were asked to vote on which saint they wanted to name the church after - there was a list of 5 or 6 choices. I don’t know if that vote was the deciding factor, or just taken as a recommendation. This was before the church was built - we were celebrating mass in the gym of a nearby school at the time.