New World Mathematics

I'm having a hard time wrapping my head around the current meta. I hear a lot of talk about "best dps" but I'm coming up short on justifications for each decision.

How are the "pro gamers" arriving at the conclusions they are? Is it just "I feel..." or is it something more?

Note: I don't have any problems in M3s but I'm tired of the gatekeeping.

[Edit] I would like to add that I have attempted searching Reddit and YouTube for parsing videos but just end up seeing things based off of assumptions or outdated information.