Location of files: Main Storage versus SD card
What's in which directory?

/sdcard is the so-called "internal SD card" (internal-sd, on "internal storage" and not physically removable without breaking the device, hence also referenced as "Main Storage" in your case), while /storage/&lt;card_id&gt; is the physically removable external-sd.

are there other places data might be stored?

There are. Without root, you e.g. won't have access to where your apps store their data (/data/data/*). To perform a real full backup, you'd need a custom recovery like twrp to perform a nandroid backup. The closest you can come to that is adb backup – but even an "adb full backup" is incomplete (especially as apps can "opt-out" of that, so they won't be backed up by adb backup).