Okay, that explains it then. With the Battle Pass, they give you Blueprints for all types of guns as you progress through the Tiers, which you do simply by playing.
The easiest way to describe what Blueprints are is to look at the guns you find in Warzone with actual names. Like the Oceanographer, Soul of the Beast, Glitch, etc. Those guns come equipped with certain scopes, mags, designs, etc. instead of being in base form. Those are Blueprints.
So that's what you get for certain Tiers in the Battle Pass. So you could bypass grinding some guns that might be on Level 6 with a Blueprint that comes equipped with Level 50 attachments.