In Returnal most of your progress gets lost when you die, but you can unlock Permanent Weapon Traits (Mods) to help you in the future.
First, it’s worth noting that all weapon drops are random in Returnal. You always start with a weak pistol and you can find other weapons in chests and from defeating miniboss (or main boss) enemies. But it’s always a surprise what weapons you’ll randomly get. Each weapon also has 2 variants, the variants have different traits. One variant of a weapon might shoot quicker but deals less damage per hit and the second variant might shoot slower but deals more damage per hit, stuff like that.
Once you find a weapon, pay attention to the progress bar in the bottom left corner of your screen above the health bar. This shows how close you are to unlocking the permanent weapon trait. To fill up this bar simply defeat enemies with your weapon! That’s all there is to it. Just killing enemies with a weapon will unlock the permanent weapon traits. It takes around 10-15 kills to unlock the level 1 weapon trait. Higher Level weapon traits will require more kills but those aren’t available until the 5th biome.
You can also view your traits by pressing Options-Button > Equipment > scroll down to the traits below the weapon stats.
When you reach new biomes, new weapon traits will also become available. Not only will there be more weapon traits per weapon but also higher level traits that increase the effects. This happens automatically by just advancing through the story.
Again, keep in mind that each weapon has 2 variants that have different permanent traits! For example, if you found Variant 1 of the Shotgun and unlocked its permanent traits, you might find Variant 2 on the next run but to your surprise none of its traits are unlocked. That’s because it’s a different variant of the same weapon. This can be a bit confusing at first. Unfortunately, there’s no way to swap weapon variants, you have to get lucky with the random drops to get what you want.