It very much depends on how you are uploading, and where the images currently are on your system.
If uploading by syncing from LR Classic, the existing images on the hard drive will remain where they are, but note that Classic only uploads Smart Previews of your originals, so you wouldn't have the originals in the cloud.
If uploading by importing the originals into the LRCC desktop app, LRCC copies the originals into it's own local library space, from where they are then uploaded to the cloud. So the original files (if not on a memory card of course) remain where they were on your hard drive (but are not subsequently referenced or used by LRCC). However, you can also instruct LRCC to store its own local copy of the originals, see the Local Storage tab of the LRCC Preferences)....if you elect not to use that option, the local copy that LRCC makes during import will become eligible for deletion after the successful upload has been made.