Why on Earth my LIbrary is faulty? How to tag?

Ok, probably because I am a senior citizen (my excuse) but I cannot see why so many of the albums in my music library do not come up in searches and so many others do. I think that is because some tags are missing, but how do I get the tags added? Do I have to do this outside of kodi?
1. I know how to add music folders to the library
2. I have all my music on drives connected to Rasp pi2 via usb
3. I have set them to use universal album scraper when I scan the contents of the drives to the library
4. When I select one of the drives (and all the music folders on it) and hit scan to library, is this adding all the tagsConfused? Obviously not because it is hit and miss which ones will be later found in library searches.

Do I have to add the drives using “add music” and then scan to library?
I have read the wiki and so forth but am still having a series of “senior moments” . I would be grateful for advice on how to get tags read and added to my files on my hard drives by using kodi online so that they then will show up properly in libary searches.
Thanks so much.