It’s not spreading though.

I live in a small town in Guatemala and the mormon church here is growing very rapidly. According to Google, they're growing at 1.5% annually, which means 255,036 new Mormons every year. However, a church with even a hundred people can heavily sway a small, remote community.

I agree it's not "spreading" per se, but their numbers are respectable among proselytizing religions today. When Islam reached a 1.8% growth rate in the 2000s we were inundated with proclamations that it was the fastest growing religion in the world (eg: https://foreignpolicy.com/2007/05/14/the-list-the-worlds-fas...).

In some places it is more effective than others; Mormons are very present in the Pacific islands, for example.

I know that they go on missions. My question was regarding the "Mormon-American" framing which suggests that Mormonism exists in other countries, which I was not aware of.

Very much so, in fact, more Mormons live outside of the US than within the US. The church reports a population of 16,663,663 with 6,592,195 of those living within the US. (got those numbers from Wikipedia).

This article from 10 years ago reports that 56% of Mormon live outside the US and Canada at that time- https://www.deseret.com/2013/8/30/20524833/the-majority-of-m...