While playing either Operations or Eternal War, you'll unlock cosmetics for your bespoke Space Marine, as well as experience and "armory data" to spend on skill and gear upgrades. The six playable Space Marine classes have their own skill trees, while it looks like individual weapon types have "perk trees" of their own. Additionally, as you use a weapon, it seems like you'll unlock modified variants of that weapon archetype. A "master-crafted" auto bolt rifle shown in the video had increased accuracy compared to the default variant, but decreased magazine capacity and ammo reserves.