Images are only referenced back to the original folder. The Lightroom catalog does not contain any images. It is a database that contains references to where images are located in folders on hard drives connected to your computer as well as all of the adjustments and organizing into collections and keywording that you have done with those images. Additionally, Lightroom does not make adjustments directly to the master images. There is an option available that you can choose in preferences that will cause Lightroom to write metadata changes to the file (XMP files in the case of raw files), but if that option is not selected then changes are only written to the catalog and the only way to have an image with all of the adjustments applied to it is to export a copy. That is the way I use LrC.
Lightroom records all the work you do with image files in the Lightroom Catalog it is strictly a database file that only contains information. The Lightroom Catalog File does not contain image files or copies of the image files. At import Lightroom creates Previews, these are proprietory files which assists the application in displaying the image files on the computer.
Your original files are contained in folders on your computer internal, external, network disk drives. You need to have procedures in place to maintain backups of the files.
It’s important you become very familiar with how Lightroom Catalog file functions to avoid making poor decisions in your workflow.
It should be clear from the earlier replies that imported files are always referenced. However, there can still be some confusion for beginning Lightroom Classic users as to where that reference is. This is because you can customize the import destination in the expanded Import dialog box. Not only do you get those options to Add (reference in place), Move (change folder then reference), or Copy (reference the path to the copy), the Destination panel lets you customize where Move and Copy imports go. If you have changed the options over time, imports may have gone to different folders and subfolders, maybe even to different internal or external volumes.
If you ever wonder where a photo is really stored, you can find out in one step. With a photo selected in Lightroom Classic, choose Photo > Show in Finder/Explorer. That will switch to your computer’s desktop and open the photo’s containing folder with its file selected. (You may find it faster to choose that command when right-clicking a photo instead.) You will find that every photo in your Lightroom Classic catalog exists somewhere on an internal or external volume of yours, depending on how your Import options were set up.