Discussion about the purposes of Dell server hardware

Hello everyone who is reading, I joined this community to learn more about the theoretical side of servers. I have a few questions to ask, and I hope someone can help clarify my doubts.

I mainly work by assembling and testing servers throughout the day. Some parts pass through me, and I find myself wondering about their function. Of course, I’ve already researched, but I’d like to hear the perspective of someone who is an expert on the subject.

What is the function of an HBA card? What is a storage used for? How do you choose between a server and a storage?