They just dominated the game without having good enough chances to shoot, that's the reason why they didn't have even 1 shot on goal out of the 7 shot attempts they had. Otavio played an important role in Arsenal's failure to create good opportunities, you can see how he performed in the match, he even got the highest rating and MVP in this match.
The result of this match didn't really surprise me because Porto played at home, there was still the second leg remaining and the match would take place at the Emirates, the only thing Arsenal had to think about was how they would break down Porto's defense if they still wanted to progress to the next round.
Of course Agg 1 goal is not a difficult thing for Arsenal to chase, especially in the second leg, Arsenal will play at home, but what happened in the first leg is something that is very regrettable.
Porto played extraordinary, they seemed to be able to cover Arsenal's every move and the Arsenal players were unable to develop their game.
And in the other match, Barcelona drew at home to Napoli, this was in line with predictions and Barcelona quite benefited from the result of this draw because it would be easier for them to look forward to the second leg which will be played at Barcelona's home ground.