Which version of Lightroom are you using?
Have you restarted your computer and let the system reconnect to the physical location of the deleted files, so that it can update the catalog with that information?
Also, this discussion How do I delete the jpeg from raw+jpeg imported images? is somewhat related to your query, please reviw it and let us know if that helps.
I don't completely understand your post.
Can I change the extension in the Lightroom Classic CC to DNG since I (accidentally) deleted all JPGs from the folder in Finder. So I've imported them from camera as DNG+JPG, but now I basically have only JPGs, but the Lightroom is still showing DNG+JPG.
If you originally imported both DNGs and JPGs and then Deleted the JPGs you shouldn't reimport the JPGS. You should of just Copied them to that folder. Or opened the Trash and put the images back into that folder from the trash.
What you are now looking at is "Previous Import" in the Catalog section of the left hand panel in the Library module. With that selected in the Catalog section all that is shown is your last import. In this case just the deleted JPG images you were trying to replace.
Personally if I import both RAW and JPGs of the same shot I have the option checked to treat JPGs as separate files. That way I can see them in the library and develop modules instead of having them hidden. IMO the purpose of LR is to catalog and display all your images. Not just part of them.