edit plugin add functionality

Hello. To edit and change the plugin. For example, add a field to the table and save and extract the information in the front and back end. In php with the commands alter table, insert, update, select the commands mysql and echo in the duplicate loop SQL query results can be done. In WordPress, exactly what methods, classes and functions should be used and how. For example, we have three tables of student, teacher and class. And we want to add a payment field and payment date to the class table and use it. In the pages of student and teacher and class and school principal. With functions The field of payment and its date are metadata or classification that are not and are not related to posts. It is related to the three fields id of student, teacher and class. Does not the field of a table have to be post, metadata, or classification?