Exploring codespaces as temporary dev containers (via) DJ Adams shows how to use GitHub Codespaces without interacting with their web UI at all: you can run “gh codespace create --repo ...” to create a new instance, then SSH directly into it using “gh codespace ssh --codespace codespacename”.
This turns Codespaces into an extremely convenient way to spin up a scratch on-demand Linux container where you pay for just the time that the machine spends running.