Creative Assembly uses a modular content system. When you download Total War: Warhammer 3, almost all the DLC data is already on your hard drive so that you can face those factions in the campaign. The DLC purchase simply acts as a "key" to make those factions playable for the user.
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When the game launches, it queries the wrapper instead of the actual Steam server, receiving a "Yes" for every ownership check. The Risks: Security and Stability Creative Assembly uses a modular content system