Downloadable .dll files from unofficial sources are common vectors for trojans and other malicious software.
While some users on forums like Reddit claim they have used these tools for years without issue, the risks are real: planet zoo dlc unlocker work
If you want more animals but don't want to risk your account, there are legitimate ways to expand your game: Do you REALLY need DLC in Planet Zoo? Downloadable
You rename the original steam_api64.dll and replace it with a modified version that validates all DLC requests automatically. The Risks of Using Unlockers The Risks of Using Unlockers Technically, Planet Zoo
Technically, Planet Zoo includes many DLC files within the base game's installation folder (specifically in the OVL data folder). Because the assets are already on your hard drive, an "unlocker" doesn't download new content; instead, it uses a custom (like SmokeAPI or CreamAPI) to trick Steam into thinking you own the licenses for these packs.
Game updates often break unlockers. For example, some versions of CreamInstaller were reported to break Planet Zoo after certain patches. Better Alternatives to Unlockers