An application will have more than one page. You can use the Application menu to navigate between pages, or access the API page. Multi-page apps will be covered in detail in the Structuring an Application chapter.
Bubble saves your app automatically, in real time. As soon as you modify an element, action, setting, etc., the change will be saved on the server. You will see a 'saving' notification in the top bar for a brief instant, which will change to 'saved' as soon as the saving process is finished.
Note that sometimes the notification can turn red, usually because the internet connection hasn't been stable for some time. When this happens, you should stop working on your app for a bit and see if the connection returns to normal. If it doesn't, refreshing the editor is the best way to get back to a normal state. You may lose the last changes you made when you refresh. Do not keep working on your app if the notification turns red, as you may lose any changes made to the app after the color change.
All modifications to your app, within one browser session, are undoable/redoable. You can click on both icons in the top bar to revert changes or apply them again. Note that the changes can happen in some parts of the editor that aren't visible on the page you're on in that moment, so use these buttons with caution if you don't see what they are doing.
You can click on Preview at any time, and that will run your application on the page you are currently editing in the editor. As changes are saved in real-time, the version you see in the run-mode tab is always the latest version of your app (after you click the 'Preview' button). Note that when you have a run-mode tab open and you modify your app, a notification will appear as soon as the version running is older than the version in the editor. Clicking on that message will refresh the page and run the latest version in the run-mode tab.
More than one user can modify an app at the same time (note that you need to be on a paid plan to invite users). When someone else is modifying the app you are in, you will see the other user's mouse moving. This is useful to prevent two people from modifying the same element at the same time.