Reading and sending emails GMail API + PHP, maybe?

Question

Google

Good afternoon !
There is a domain, mail for which is hosted by Google.
As I understand it, using the GMail API, you can connect to a mailbox from PHP and show the necessary letters from there or send a new message.
- enabled the API from my mailbox (is it possible to do this for the entire domain? how?), received the keys and the json file.
- downloaded Google Client Library
https://developers.google.com/gmail/api/quickstart...
https://github.com/google/google-api-php-client
- uploaded to the root of the site
How to get a list of letters?
require __DIR__ . '/vendor/autoload.php';
where to get this?
Thanks a lot.
Best regards, Sergei.