If you want to get information about how an arbitrary website (or the map, visualization etc. they contain) was created, it is helpful to view the source code of this site.
In most browsers, you can to this, by a "right click" on the page and then select "View Source".
The source code of your site looks something like this:
Other maps might origin from web map services (WMS), they are often included as images.
Some of the most common visualization libraries (for maps) are OpenLayers, Leaflet, GoogeMaps API and also d3js.