Points are rendered server-side, backed by Elasticsearch, and served as PNG tiles for each zoom level. Individual markers are only rendered for small sets. Some of the relevant source code:

https://github.com/inaturalist/inaturalist/blob/main/app/ass...

https://github.com/inaturalist/inaturalist/blob/main/app/ass...

https://github.com/inaturalist/inaturalist/blob/main/app/ass...

You may want to look into the PMTiles format and tippecanoe. It efficiently produces pyramidal XYZ tile overviews of vector data. Sometimes this is also done server side via the PostGIS asMVT ffunction, or Martin.

For client side rendering, deck.gl is quite good, also a newer library called lonboard from DevelopmentSeed.