Question about painting models and the importance of pauldrons

First and foremost, I'm looking to buy primaris marines to paint as ultramarines or to buy pre-painted ultramarines that are exclusively primaris marines. I'm looking at a great set of Marines that all have red pauldrons

1. Guilliman is in the set and I'm wondering if the red pauldrons are related? (I don't have any space marine codices only chaos codices so I'm not familiar with ultramarine company markings)

2. If the red pauldrons are related to a particular company, what company and what markings are associated with the pauldrons for veterans, unit designations, etc etc

(The pre-painted Marines I'm looking at have red pauldrons on both the right and left pauldrons if that matters or helps to identify the company)

(Looking at the rules I hope I'm not in the wrong sub, I genuinely want info on ultramarines for the purpose of painting more than the purpose of commerce, if I'm in the wrong sub please I'd love some suggestions on where to ask about this, thanks in advance)