I know what a group being open means in the later parts of a group draw, normally teams of the same country cannot play in the same group. But I find it very peculiar on the first pot, in other words the first teams to be placed to groups some groups are not "open" to a club. Does that have anything to do with the game schedule Tue/Wed and TV viewership? So that for instance Real Madrid and Barcelona do not play the same day? Or is it a completely different story?
This is due to seeding - the top seeds all avoid each other in the first round, in a similar way to other sports such as Tennis.
In your example, Real Madrid & Barcelona are both top seeds so cannot meet in the group stages.