In the ancient KiCad V4, KiCad only had links to external libraries for the graphics of schematic symbols, but that has been improved gradually. In KiCad V5 this info was cashed in the [Project]-cache.lib as a temporary measure, and people often mistakingly deleted this file, did not back it up nor put it in git repositories, because they did not know it's meaning, and KiCad could reconstruct the project as long as the original libraries were still present.
In KiCad V6 and onward, all the graphics of schematic symbols are stored in the schematic file itself. (Footprints were already cached in the PCB file on older versions). The result is that KiCad projects have become more robust because they are independent of external libraries.
If you wish, you can create a library with all used schematic symbols or footprints with:
Schematic Editor / File / Export / Symbols to New LibraryPCB Editor / File / Export / Footprints to New LibraryBut it's not mandatory. Keeping the links to external libraries may even be beneficial.
KiCad V7 does have a new ERC check for verifying used schematic symbols with their library counterparts, and it's up to the user whether to update the schematic, or silence the ERC for this.
In the KiCad V5 to V6 transition, the schematic file format was completely rewritten, and it was a quite big change, although most users did not notice what was happening under the hood. The transition of projects from KiCad V6 to V7 should be trivial.
To me, it looks like it's a bit too early to adopt v7 in any professional setting.
I agree with that. I also have the feeling the release of KiCad V7 was rushed a bit. If your business and income depends on KiCad, then it is better to wait until V7.0.1 is released, or maybe even V7.0.2. (And this is true for all mayor KiCad version updates) KiCad's development is going quite fast in the last few years (V7 has around 70 new features, developed in just a year) KiCad has to release it as a a "stable" version to get many more users to adopt it, and help with exposing bugs. As an example, just today a quite serious memory leak was reported for V7:
https://gitlab.com/kicad/code/kicad/-/issues/14066 And this hole got plugged in 37 minutes after reporting it, and it will be included in KiCad V7.0.1
When you use KiCad, you should always keep it updated. Changes in the third digit are bug fix releases, and updates should always be safe. These bug fix updates are released approximately once a month. (But again, if your ability to buy sandwiches and pay your mortgage depends on it, wait a few weeks)