To start recording, use the Record button.

You can click the Stop Recording icon on the recording control widget.
You can click the Stop Recording icon on the menu bar icon.
If you have a MacBook with a screen notch, this icon may not be visible.
You can right-click the Screen Studio dock icon and click Finish Recording
You can also use the global Start / Stop shortcut you set during the onboarding.
You can change your global Start / Stop shortcut in the app settings.
After you have finished recording, the editor window will appear, allowing you to edit your recording.