These were “rare” in that there were few students selected for each position each summer.
For the European Commission position about 6-8 would apply and 2 would be selected each summer. For the BoE position, I’m uncertain as to how many applied each year with 1 - 2 selected.
However, there were many similar programs offered. Before the plague years (suspended for the pandemic) Yale, for example, would have special (funded! with Yale arranged housing provided) opportunities in major international cities around the world including Paris, Brussels, London, Tel Aviv, etc. While only two of the positions in Brussels were at the EC, there were also positions at the ICC, and other very special programs.