What a farce this is turning out to be.
Qualifying was from ten groups with either four or five teams in them. The top two from each group qualified for the finals, but as that give a weird number the teams finishing third are going into play-off games in order to get 24 in the finals.
However, 24 does not divide by two to get 1 final, 2 semis, 4 quarters and a last sixteen. So, UEFA has come up with six groups of four, but as that also does not divide properly the top two from each group go through as do four of those finishing third. So a team that did not 'qualify' in the traditional sense and then did not 'progress' in the traditional sense could get lucky and go on the win the thing. Dividing the 24 into four groups of six with the top two from each group progressing to the quarter finals was just too bloody easy.
The next mess is that the competition is not being held in one country or two neighbours, it is being held all over the place. This means that each group has host cities and some teams will play all their games in host cities. England, for example, will have huge home advantage as all their games will be played at Wembley as will the semis and the final.
The principles of quantum physics are easier to understand.