Downloadable .dll files from unofficial sources are common vectors for trojans and other malicious software.
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. planet zoo dlc unlocker work
The unlocker "hooks" into the game process to intercept license checks. Downloadable
While some users on forums like Reddit claim they have used these tools for years without issue, the risks are real: an "unlocker" doesn't download new content