Author here! Thanks for posting the app!

Two notes on what you are looking at:

Not every vehicle is a live position. STIB-MIVB and TEC publish vehicle positions and those are shown as received. De Lijn and NMBS/SNCB do not, so their vehicles are computed from the timetable and corrected with the delays they do publish (GTFS-RT). Each marker is tagged live, computed with delay, or computed, and the computed ones are drawn dimmer.

All data is open data. The four operators publish through one federal portal (data.belgianmobility.io) under CC BY 4.0: GTFS for timetables and route geometry, GTFS-RT for delays, alerts and positions where available. NMBS ships no route geometry, so train paths are routed over Infrabel's open track network.

This is not specific to Belgium. Nearly every country publishes public-transport open data in the same formats, and I was able to locate feeds for most European countries. A map like this can be built for almost any of them!

Happy to answer questions on the data or the simulation :)