So, once upon a time I worked for a town engineer, and had to learn about FEMA maps.
EVERYONE is in a flood zone. The entire country is mapped out - and there are several different zones. All land falls into one zone or another, including "undetermined" flood risk areas. The phrase "I'm not in a flood zone" is basically a misunderstanding of flood zones.
A floodplain is a flat area of land next to a river or stream. It has nothing to do with how many feet above sea level you are. If a river runs through a mountain, and you live at the top of the mountain, and the river is next to your property, then you might be in a hazardous floodplain and need insurance, even though your property is over a thousand feet above sea level.
The actual zone letters are what FEMA uses to determine the risk of flood hazard.
The above is very basic, "explain it to me like I'm 11 years old" language, and there's a lot more technical stuff behind it, and insurance adds more layers of nuance.
You can see a more complete explanation here:
FEMA Flood Maps and Zones Explained | FEMA.gov
and you can click on the links on that webpage for details on specifics.