It is my understanding that currently it’s a bit of a mess. There’s a standalone kit, but also each app contains its own version of kit (under kit subfolder).
Looking at the current latest versions available on Windows for major apps:
Kit Version: 100.1.45348-0d7f253d-release
Looks like the newer the app release, the more up to date kit it ships with. I am not sure about how it will be consolidated going forward.
Note that I am by no means an expert on this, but it looks like If you’re interested in working with the latest kit version to create apps and extensions, you can pick the kit version you like from the available apps.