Yes, they are. Many of them are arrays. For example, recipient.name will be a customer, supplier, payer, payee, employee, or a blank, depending on the form being displayed. And cell.text can be a date, item, description, quantity, unit price, amount, tax code, etc., depending on the context. The program fills the array according to what data must be shown in a table.
Understand that themes do not use the same variables as may be present in the database. They use a set of variables necessary for whatever is being displayed. Every built-in theme includes the same variables (except for the emphasis.text variable used to display PAID IN FULL and OVERDUE stamps on some sales invoices). That is because a theme can display all transaction types. Data from the database is mapped into the Liquid theme variables by the code that defines the particular transaction being displayed.
My guess is that you are looking for specific database variables to populate a theme. That is not how themes work. You can only use theme variables. As I already said, they are all there, and there are not very many because they are arrays instead of individual values.