Hi and welcome!
There are many different ways to collect, everyone is slightly different. Personally I do both of what you mention - I focus on collecting a team (West Coast obviously) but also love opening boxes and packs, because in my opinion that’s the whole fun of it!
I can then trade what I get for Eagles cards (or if I hit an Eagles card straight out of the pack even better) or if I can’t find a trade, sell the card and then use the money to buy some. You will be stuck with commons, I’ve got several storage boxes full. Sometimes if you have a full set from a box for example you might be able to sell it, while I know some people give them away to schools/children’s hospitals/charities.
My advice would be to dive into it without thinking too much, you’ll quickly find out what you enjoy most!
Cheers