What is $cman

Your approach is incorrect, do not put the horse in from of the wagon.
What problem do you need to solve or what you trying to achieve?

Exploring the Massmailer template I found that the global variable $ cman is used to find the fields in the table from the SQL query to be used in the mail composition.

Because before I use something I have to learn how it works so I want to read the documentation of the variable.

You cannot use anything that is not in the documentation. The internal variables are subject to change without notice.
Hence the question - what exactly do you want to achieve?

I have created a table with 30 text fields, 30 number fields, 30 boolean fields, 30 dates and another text field that is for grouping.

I want to create results from queries in a table or joined tables or even to use some fields from this table to group them as a pseudo table and displaying only the non-blank fields.

There is no logic to what I want since I could use the SQL query directly but my employees want ready-made lists to work with. I don't know how much longer I will be able to support the application I made and add options to the menu.

So I want them to be able to add new items in a form like tasks in Mass Mailer template and select them from a dropdown list. It may be too much for me but it's worth a try

I'm sorry, but you are not saying what exactly is the problem you facing.
Are you asking how you can execute a SQL query in your events?