First thing....no, you cannot "link" LRCC to the set of originals of your external drive.
Basically, when you "import" photos into LRCC it first of all copies them into its own library space. It does this so that they are protected whilst they are being uploaded to the cloud, and the default library space in on the user's system drive. However, you can specify a different drive location for those copied originals if you wish.
So, when the upload has completed you will basically have three copies of your original files: the originals on the external drive (which LRCC totally ignores after the import), the copy that LRCC has stored in it's local library, and of course the third copy (which LRCC regards as the "master") will be in the cloud. The copy in the LRCC local library may over time be removed, how quickly depends on the amount of free space on the drive and the settings on the Local Storage tab of the LRCC preferences.
If you delete the any of the files in LRCC's local library, there will be no effect on your edits, provided the upload to the cloud has completed before you start deleting the local originals. As I said, once uploaded, LRCC regards the cloud copy as the master.
Before you experiment any further, make sure you have your photos backed up. You need to be certain in your own mind that you won't lose anything if you make any mistakes from not understanding "Lightroom CC".
"Lightroom CC" is cloud-dependent. That's its concept, so folders on your local system can't be linked, and once photos have synced to the cloud it considers them to be the originals. Any local copies are only kept for convenience - eg if you work offline.
When you write about linking, you are probably thinking about the Lightroom that has been so successful for 10 years - that's now called "Lightroom Classic". It may be that Adobe's product names have misled you.