So where are they? Are they in the cloud or something? Scared and confused.
This will depend on the Photos > Preferences > General you used.
Photos has two different ways to add photos to the library - by default it will copy the photos into the Photos Library (managed library) and the original photos will be stored in the Masters folder, but if you change the default, it can reference the photos in the original location (referenced library) without copying the photos into the library, see this Help page:
https://help.apple.com/photos/mac/1.2/?lang=en#/pht12e7a8015
If you enabled Photos > iCloud > iCloud Photo Library, photos will have uploaded to photos to iCloud Photo Library. And if you also enabled "Optimize Storage", Photos may have removed originals from the Masters folder to save storage. Inthat case you need first to download the originals from iCloud back to the Photos Library.
In any case, put the Masters folder you removed back into the Photos Library, so Photos can still use the library and you can use it to find the originals. But first we need to know, his you are using iCloud Photo Library, and if you imported the photos to Photos with the "copy items into the Library " option or idk you are using a referenced library.