Real odds, from the game's own code. Chances are worked out the way Rebirth's loot code rolls them, then checked against 100,000 simulated opens (they agree within a quarter of a percentage point). They assume default settings: 100% loot abundance and ammo, and no looting perks. Your loot stage changes most of the numbers; it rises with your level, the biome you're standing in, and the POI's tier (use Work out my loot stage). "Chance" means per container opened, per zombie killed, or per trader restock.
Search for an item to see every way to get it.
Pick a container to see everything it can hold.