An article of rbb (German language) gives an overview and provide details about Euro2024 fan zone at Brandenburg Gate and other public viewing places in Berlin. I do not know if the mentioned restrictions about fan zone Berlin will also be carried out in the fan zones of the other nine host cities.
Except, translated with DeepL:
"Admission to the Fan Mile is generally free. No tickets are required. However, minors under the age of 14 will only be admitted if accompanied by an adult. The maximum capacity of the fan zone at the Brandenburg Gate is officially 30,000 people. If necessary, the area can be extended to accommodate over 100,000 spectators. If it is too full, the “one in, one out” principle is applied, whereby one person is allowed to enter for every fan who leaves the fan zone. This involves corresponding waiting times.
Backpacks, suitcases and bags larger than A4 are not permitted. Only non-alcoholic drinks in plastic bottles or Tetra Paks up to 0.5 liters may be brought in. It is forbidden to bring alcoholic drinks and glass bottles. There will be checks on persons and bags."
Funny side info: Also movie Bad Boys Will Smith and Martin Lawrence are learning German football fan songs (video). Hope no geo-fencing.