I think the difference is that hitting "." as mentioned lets you browse in VSCode for free but does not launch an entire codespaces VM.
Yes, it seems like a free "Lite" version of Codespaces for every repo whether the repo has paid for and enabled Codespaces or not. It seems to be using the recent "Virtual Workspaces" support in VS Code, just using a browser-hosted VS Code rather than a local install, so some of the VS Code extensions supported in Codespaces won't work in this GitHub.Dev "every repo" version.
Thanks
Codespaces clones your repo to a VM that you have full access to, github.dev is “serverless”: it operates in your browser and all interaction happens via the GitHub API