As rkv said, that's a list built in React. The elements (store locations) in the list are filtered by what's in the map's viewport. There are also mouse event handlers on the list and on the map which trigger interactions between the two.

This site receives all its data via vector tiles [0]. So as you pan around, you fetch basic visual map information (what the geography looks like) and McDonald's data at the same time.

[0] https://docs.mapbox.com/vector-tiles/specification/

Something separate (ie. custom code). You can use "Inspect Element" in your browser to confirm.