When you press dot key in any github repository github changes to web editor mode. That's all. :d
GitHub Codespaces is a service that provides a fully cloud-based integrated development environment (IDE). It allows developers to quickly develop any repository on GitHub from any device, anywhere. GitHub Codespaces provides a virtual machine that reflects the developer's local machine settings as their development environment. This allows developers to write code from anywhere and any device. Additionally, it allows multiple users to work in the same environment, making collaboration easier for teams.
For using the two together, you should press dot (key) in any repo. Than you should open the terminal in the web editor mode. Since we can't use the terminal in the web editor, we have to open a codespace.