Custom column definitions and values are stored in the metadata database (metadata.db), they
cannot be edited directly.
The definitions are stored in a table called custom_columns
The values are stored in 'custom_column_n' tables, where 'n' is the row-id in the custom_columns tables
Tag like columns such as your "finished" column would also have a 'books_custom_column_n_links' table to associate specific tag like values to specific books.
You add new values to a tag-like custom column by entering it in a book's metadata, once you've done that for one book, you can apply it to other books.
BR