it works within the limits of the API – meaning that each CSV file returned will be limited to however many records can be returned per page (above that limit, you would have to combine pages/results), and the total number of records will be limited by the system’s general record limit (10,000 records).
this kind of visualization can be done without downloading all observations. just overlay the observation map tiles on top of whatever other layers you want to use. for example, here are mountain goat observations in the US over a topo map: https://jumear.github.io/stirfry/iNat_map.html?view=elevation&taxon_id=42414&place_id=1. if you click on the markers, you will get the elevation, according to USGS. see https://forum.inaturalist.org/t/in-pursuit-of-mappiness-part-1/21864 for other examples.