kubuntu does not index files by default, you need to enable it.
also enable file contents to get at the metadata, but do NOT enable hidden files.
as long as your files are not tucked into a hidden directory they will be indexed and the contents will populate your columns in dolphin.
it may take a day to fully populate as baloo (the file indexer) is designed to run in the background so as not to be intrusive.
if you are using snap in kubuntu, i also recommend that you add the ~/snap folder to the folders section and mark it as Excluded, since there is nothing in there worth indexing and it’s not automatically excluded like all the hidden folders.