Building a hierarchy
A simple diagram in order not to get confused. In short, at the lowest level odoo operates ir.ui.view it can be both standard list,form and qweb.
Next is the report, it uses several views and generates a pdf from them. If this is a standard qweb-pdf report. If a custom docx, then it uses a docx file as a template instead of ir.ui.view as in this module.
Next, in the mail module, we can configure mail.template this is already directly for working with mail. The report itself can only be downloaded locally. As soon as we use mail.template, we select a report in it and can already send it by mail, there are also additional fields as the recipient of the sending server and so on. Well, the compose hierarchy completes it and we see it when we send a commercial offer for example, that is, it is a dynamic wizard that allows you to choose from several mail.template and directly send a letter.