Apple support has published a few tutorial videos. You may want to search this page for "Photos": https://www.youtube.com/channel/UCYFQ33UIPERYx8-ZHucZbDA/videos?view=0&sort=da
I prefer the user guide, however to the video clips: Photos User Guide for Mac - Apple Support
Photos is a non-destructive image editor, but you have to import the photos to Photos, see: Overview of importing .
The edits will be saved in the library and the original will remain unchanged:
Theoretically, you could import the images as referenced files, keeping the full resolution raw originals on an external volume and only the edited version in your Photos Library, but it is not easy to fix broken references. You will have a hard time to reconnect the original files to the edited versions, if you ever have to replace the external volume or restore the originals from a back-up. There are also some bugs, for example, the referenced images will not appear on the maps in the Places album.
The easiest way to use Photos would be to keep the complete Photos Library on your external volume, together with the RAW files, and to import the RAW files as managed files, copied into the library.