Scrap is material that can be used to produce parts for car upgrades. You can get scrap in battles.
All cars can carry an amount of scrap. The more you upgrade your car, the more scrap it can carry.
All scrap that you get in battle is stored in the scrapyard. Scavengers sort it and turn it into car parts. Every 100 scrap parts will be processed into 5 or 6 car parts.
All car parts produced are sent to storage. When the storage is full, scavengers stop producing new parts, so make sure you have enough space in storage.
You can upgrade the scrapyard to store more scrap and hire scavengers. Hiring and upgrading scavengers allows you to produce parts more quickly.