AlbertBel, I have started a new thread for your question. Although it referred to "at/in" as did the thread you originally posted it on, I think this should be dealt with separately.
In your context, use "at". It is clear (I think) that you are talking about football (soccer) and in that context we say that teams play "at" a specific ground or use the team name. "In Liverpool" simply means somewhere in the city.
Student or Learner