Dear all, 

I'm trying to send an invoice to the invoicee via the External API. (using this package: Ang3 php-odoo-api-client) 

How can I know which Models / Methods / Fields to use in order to send this mail, with the invoice? 

I'm trying to find out via various ways: checking the External API docs, searching through the Odoo forum, searching online (tutorials, ..). 
The External API docs don't help. 
Couldn't find a relevant / recent Odoo forum post
Online tutorials, via YouTube videos are mostly done by non native-english speaking people so hard to follow. 

I've found several things about using the wizard programmatically in order to send the mail. 
First composing the message (via the wizard?) and then sending it via an additional API call? 

I'd like to find out for versions 17, 18 and 19. 

Looking for a push in the right direction

Thanking you in advance.