@MrFoxcroft: Thanks for the feedback. If there's specifics you feel the docs are missing, we'd love to hear what would help you so we can fill that gap on http://docs.microsoft.com. Identity is a fairly lightweight plugin system for authorization.
You mentioned re folder access "That clearly is really the only purpose of the framework, but not explained." Identity's purpose isn't to provide access to folders, Windows Authentication has been the solution for years for that - since it works on the security that's already used on your folders. You can find details for example here: https://docs.microsoft.com/en-us/aspnet/core/security/authentication/windowsauth
If you want a simple, easy to configure Identity solution to enable users to add login functionality to your application through custom accounts or third party providers, then Identity works well here. It should require little configuration to get up and running, the out of the box templates give you enough to get started, we wanted to dive into customizing a bit more in the video but that doesn't mean what we covered isn't documented. Check out https://docs.microsoft.com/en-us/aspnet/core/security/authentication/identity?tabs=visual-studio
You do, but can get one cheap (Mac Mini for ex) or use a cloud based Mac provider (I haven't tried the full workflow with a cloud provider though - not sure if you can provision initially there or not)
** Please reach out to me with any questions, be happy to help! (Email me, comments here I may not see) ** At the end I showed some images of games on the platform, you can check out more details on them here: unity3d.com/pages/windows/gallery