As far as player upgrades go, there's two different types: Player skills and Weapon upgrades. Player Skills will be completely lost once your character dies. Weapon upgrades are permanent and will remain fully unlocked if your character dies. As an example to get these kind of skills, one has to kill a certain number of zombies to unlock things like faster reload time, faster draw speed, and reduced recoil among other types of handy skills to make your character stronger the longer he or she survives. As for weapon upgrades, let's say you find items you can combine at the safe house workbench to add a second barrel to a shotgun. Even when you die while carrying this newly upgraded weapon, it will remain in the area where you died. If you choose to search for it with the next new playable character, then you still use that upgraded weapon as is or you can choose to likely find another shotgun that remains in this upgraded form.