Daxter jacking scrap from an enemy plane, with scrap pieces building up in his on-screen back-pack.
Scrap metal, or simply scrap, is a currency in The Lost Frontier. Scrap is a prominent resource in areas visited along the Brink, and has many uses. The primary use is exchanging it for plane weapons and mods, though it was also used once to buy the Gunship from a pirate in the Phantom Blade's hangar. It is also particularly valuable to pirates at Far Drop, who would offer Jak Precursor orbs in exchange for scrap to repair parts of the town after the mission "Defeat the dark mutants".
The most common way to find scrap is by destroying planes or completing side missions, such as from pirate radio towers, Barter's bar brawls, and interestingly even by completing requests for Precursor statues. One minigame also offers scrap as a reward, which is the plane races seen at Brink Island and Sector Zero. Aeropans do not appear to utilize scrap as a currency, as their Danger Course only rewarded Precursor orb and it was otherwise never mentioned there.
After exhausting all the above possibilities, the next way to collect scrap is by either Daxterjacking, especially with a completely upgraded jack more scrap mod, or by flying around plane-dense locations such as the Aeropan research rig, especially with stronger weapons such as the Armageddon.