You should be using an external drive to keep your files and data backed up with Time Machine or other backup solution. To run without is to risk losing your stuff at some point. Apple has given you the application to do the job, just add an external drive that is 2x the capacity of the internal startup drive.
How to back up your Mac - Apple Support
Re: your saved files – The files you save are wherever it was that you saved them. You get to choose the location whenever you save a file. Without more details it's difficult to know how to assist. Open a new Finder window and you can navigate to all the locations (folders) in your user home folder to find what you need. Regardless of the Mac model that you use, the OS and Finder work the same as any other Mac.
Use the Finder on Mac - Apple Support