What happens at the border between Andorra and Spain?
On a Sunday afternoon after a day of shopping, trying to get back to Spain?
An awfully long queue to get through customs.
Other than that, nothing much. Most people who visit Andorra are either Spanish or French, so there's no special procedure to check who enters or not since Andorra has a special relationship with the EU.
Occasionally they will shut down the border or do heavy patrolling if there's some terrorist on the loose or anything of sorts.
There's almost no immigration checks in both sides, even Andorra being not part of Schengen nor EU. In this topic I am curious if Andorran citizen would get a passport stamp when entering other Schengen members and if they need passport to travel inside Schengen, since they aren't EU/EEA member nor Schengen, but they do have Freedom of Movement (just) to Spain and France. To Andorran non EU residence permit holders, I read once that they can lose much time in immigration (if not in France or Spain) if entering Schengen area by air , since it wouldn't be so common situation.
Curiously even rarely having checks and Andorra not requiring visa to anyone, people that need Schengen visa would need double or multiple entrance in order to visit Andorra, since all entrances and exits of Andorra are into Schengen. I don't know what are the consequences of entering Andorra with a single Schengen visa.
On the other hand I've seen in news that customs are commonly checked on Spanish side since, not being EU member and having los tax products, goods buying in Andorra are taxed like any other from non EU country. Considering tax there is very los, they know that a lot of Spaniards buy things there.