The currently open and active UI Document (UXML) asset name is displayed in both the Hierarchy pane as a root tree item as well as in the Canvas header inside the Viewport. An asterisk * next to the name indicates unsaved changes.
UI Builder will always start, by default, with a blank unsaved UI Document (UXML). This will display as
<unsaved file>*.uxml. You can start working within this UI Document (UXML) and then create it as an asset on disk later when you save for the first time.
You can open an existing UI Document in multiple ways:
You can save any changes to the active UI Document (UXML) by using the File > Save menu in the UI Builder’s Viewport pane toolbar. Ctrl/CMD + S might also work but the UI Builder window needs to be in focus. Note that the Unity project-wide Save won’t save the active UI Document (UXML) open in the UI Builder.