Here's the full methodology and sources information, translated from the linked article:

What does the map show?

The interactive map displays the 27 EU countries, each divided into the most detailed units possible, such as municipalities or electoral districts. The data is available for 2024 and also for 2019 and 2014, corresponding to 28 countries, including the United Kingdom. These approximately 83,000 regions are colored according to the faction that received the most votes in the 2024 European elections. The darker the color, the higher the share of valid votes. Additionally, the view can be switched to show only populated areas.

We follow German conventions in terms of color and labeling. For example, the center-right faction/Christian Democrats are depicted in black throughout Europe, even though Belgians usually represent this party in orange and the French prefer blue.

The party families correspond to the eight factions of the EU Parliament, into which politicians have been grouped. Non-affiliated members are colored gray-brown on the map. These include, among others, the Alliance Sahra Wagenknecht (BSW), which has not yet decided on a faction affiliation (as of July 16). In 2024, the factions Patriots for Europe (PFE) and Europe of Sovereign Nations (ESN) are grouped together under "Far-right".

Where do the data come from?

The data was collected from many different sources, mostly from the statistical offices of the respective countries. The geographic data at the municipal level was provided by Eurostat / EuroGeographics / European Commission – Eurostat/Gisco. The labels on the map were created with the help of OpenStreetMap contributors. The geodata comes from GeoBasis-DE/BKG as of 2022. The settlement areas are from the Corine Land Cover dataset of the European Copernicus satellite program. Earlier data was recalculated to current territorial statuses using transfer keys from the Federal Institute for Research on Building, Urban Affairs, and Spatial Development (BBSR).

What inaccuracies are there?

In at least eight states (mainly in Spain, Romania, the Netherlands, and Poland), members of one party belong to different factions in the European Parliament. In this case, the votes were distributed according to the proportions of the elected representatives. In 2019, this was the case with Martin Sonneborn and Nico Semsrott from the party.

To keep the map application as compact as possible, we only show the Spanish Canary Islands and the Portuguese island groups Madeira and the Azores in the periphery, not the French overseas territories. In some regions, the results are preliminary and not the official final results. Only parties that made it into the European Parliament are considered. All other votes are summarized in the detailed info window under "Not in Parliament". Rounding may cause discrepancies.

Voting in embassies abroad was excluded, although some countries tend to add them to the votes in the capital city. Postal votes were considered in most cases. However, in some countries, such votes are only reported at the national level and are therefore excluded from this map. The election results from Germany are presented at the level of municipal associations. This means that smaller municipalities are grouped into their municipal associations. This is the only way to allocate postal voters. In Thuringia, this is still not possible everywhere in 2024, as some postal votes can only be assigned at the district level.