Here is something you can try -- all my Notes are using iCloud so I haven't tested this myself on my computer.
Download Find Any File. This is an app that can search through a variety of folders on Macs that Spotlight does not search in.
Pick a long and unique text string from one of your On My Mac Notes entries. Search for that using Find Any File, search for file content that contains that string. Spotlight would normally be the search tool to use but Spotlight does not search through all the Library folders and files.
This will perhaps help you find the location for On My Mac Notes content.
Caution: there may be multiple files and folders that need to be migrated properly for Notes to show your content on a different Mac. There is the actual text/graphics etc. content itself, which you can search for and probably find. But there is also index, date, ordering, and sorting/filing information that is used for keeping track of folders, ordering, indexing, etc. Without the latter, which is probably stored separately in some other location, Notes won't show things properly. Ordinarily, Apple's Migration Assistant moves everything over that you need for Notes to work on the new computer. However since you did this "by hand" yourself, dragging folders from various Library Containers and the like, without getting all the content + indexing/sorting info over, you might currently have only a portion of what is needed to properly show all your Notes. It is analogous to emails: just moving over the content of the emails is not enough, one also needs the date, sorting, folders, indexing etc. that allows the user to display, read, search, file etc. the emails properly.
