Usualy in order to use The graph api you would need an App registration. That app registration will contain all the rquired data (client, tenant, application id, secret...)
In order to create an app registration you must go to https://portal.azure.com -> Azure Active Directory -> App Registrations and here you can create a new one. This App registration will require Some privileges to your Domain/organization. With such an app you can mostly controll everything inside the organisation via the Graph api as long as you can select the right permissions. I am not sure what permisions you would require for reading or sending emails , but you probably would need an app registration.
Sometimes you can even make app registrations to do things in the name of some users inside your organisations, like sending messages in teams or any other activity a user can make.
If you want to use Azure active diretory for your webcon user database you would have to create such an app registration as well. https://howto.webcon.com/integration-with-aad-using-rest-invoke-method-and-microsoft-graph/
I am not sure how helpful I've been, but if you have any other questions please ask them. :)