The two big homes for code. They store your project, keep a full history of every change, and let people work together without overwriting each other. Pick one to see what it's for, and the shortcuts that make it fast.
Your project lives here as a repository: every file, plus a complete timeline of how it got that way.
Every save is a commit with a note. You can see who changed what, when, and why, and rewind if needed.
People propose changes (a pull request), discuss them, and merge them in once everyone's happy.
These are sequences: pressed one key after another (tap g then c), not held down together. They work on the website, anywhere you're not typing in a box.
Both GitHub and GitLab pop up their full shortcut list when you press ?. The fastest way to discover more.
Most of these are tapped in order: g then i, not held together like ⌘C. They only fire when your cursor isn't in a text field.
GitHub calls a proposed change a pull request; GitLab calls it a merge request. Same concept: review, discuss, merge.
Pressing . in a repo opens a full web editor right in the browser: no setup, great for a quick fix.