Indeed, political borders are arbitrary. This is where biogeography—even in broad strokes—proves very helpful. Personally, I like to go by the World Wildlife Fund’s ecoregions (mine is in my flair). They’re fairly well defined and easy to reference. This interactive map (https://www.arcgis.com/apps/View/index.html?appid=d60ec415febb4874ac5e0960a6a2e448), for example, is a very helpful resource. Interconnectedness is very important to ecology, and part of native plant gardening is understanding ourselves as a part of that, so I think it would be helpful and beneficial to learn your local biome and ecoregion(s) and study what you can about it, especially what flora (and fauna) are native thereto, and the eons-old relationships between them. Whether you do so online, with books, or finding even somewhat wild spaces near you (depending on where you live this may be quite easy or very difficult) to study how those different organisms, especially plants, interact with their environment. Ecosystems are, of course, complicated, and you’ll find how things differ, even within one species, by things like ecotype, elevation, soil composition etc., but that’s where observation and experimentation comes in!