It’s really not that complicated. All new updates like City Updates and World Updates are free and optional downloads that are always available in the marketplace to download for those who want them. That’s why they are released on the marketplace instead of a mandatory update.
The mandatory update when you first launch the sim is usually the marketplace update, allowing you to access the new updates via the MarketPlace. In fact these are the only mandatory updates, Sim Updates and the Marketplace updates.
The other updates you’re seeing in your Content Manager are all packages that have to work and play nicely with the new WU updates. These packages have to be updated/downloaded in order for a seamless blend of the new WU and the old WUs.
Not seeing how this is too complicated. Go to MarketPlace for the new content, download updates from content manager, restart sim. Done.