On paper, Atalanta is the underdog facing Arsenal or another team. But football isn't played on paper. The Europa League victory of Atalanta was not luck. They outplayed Leverkusen, were strategically smart, resilient, and more than just good
It's true if we consider the famous case of Sevilla that won many Europa League titles but they never shown well in Champions League. The same thing can be seen with Atalanta and Leverkusen as quality and competitiveness levels between Europa League and Champions League are very big and it's not easy for an underdog can go far in Champions League. Sevilla is not an underdog but they are great example that succeed in Europa League does not mean you can repeat it at a higher level in Champions League.
Atalanta has experience; they have tactical nous; they have that drive to prove themselves on the largest arena. Undervaluate them at your own risk. This is about who has the greater heart and the better game plan, not about who has a bigger name. And Atalanta had both in spades
Atalanta some seasons ago had experience in Champions League too and they actually made impression in that season. However, some years can change many things positively or negatively and even they continue with improvement, succeed in Champions League is not easy and requires many things come at the same time.