Your Lightroom-Classic CATALOG is a database, and like all databases it only shows what you put into it.
To view your photos in the LrC Catalog (called previews) you need to ‘put’ the photo information into the catalog database: (Lr-Classic can only work with a Catalog.)
1] From a camera card- Import by the ‘COPY’ method so the camera files are copied to a hard drive folder, AND they get referenced in the catalog database. (ie. two actions are performed)
2] From your connected hard-drive (internal and external)- Import with the ‘ADD’ method. This leaves the files exactly where you want them stored and only makes the references to them in the catalog database. (ie. one action is performed)
You don’t get them “ off of the folder on my hard drive”- ADD simply leaves them in that folder and adds them to the database. COPY will, as the name suggests, make another second copy of your files placing them in another location (that the catalog now references) that confuses so many people. (The common question “where are my files”?)
Collections: Do you have a music database, or even a shelf of LP records? Do you make lists of your favorite songs, do you make playlists of your songs?- This is akin to making Collections in LrC.
Your photo files ‘live’ in a FOLDER
You make selective lists of images in a COLLECTION.
Yes, “Albums” are the equivalent of Collections, but the term “albums” is only used in the mobile Web versions of Lightroom- a distinctly different app than Lr-Classic.
You don’t “Move” photos to a Collection. You add them to a collection- just like you might add grocery items to a shopping list. And because Collections are virtual you can have the same photo in several Collections. (A shopping list for groceries. A shopping list for Hardware.)
Collections in Lr-Classic do not use any hard drive space (Folders do). Collections are only visible when you have a catalog open in LrC. Collection lists are only ‘saved’ in the catalog.
For your editing/culling you can make Collections of your ‘Selected’ images (that 100)- These you might then edit for the job, but all the 600 photos still remain in a folder.
Go get and read the Lightroom Classic Quick Start book here-
https://www.lightroomqueen.com/