symfony 4 - Multiple d'authentification : Good afternoon,

Here I am working on Symfony 4 and on the Symfony Security module.
I need to customize the authentication of my users without using plugins such as Fosuser.
This is for the following reasons:

The user connects to a "main" server (ldap) with his login/password
I need to be able to retrieve his credentials to test his account on other ldap servers in parallel.
When the user is authenticated on the main one, the user is redirected to a page telling him if everything is ok or not on the other servers. In case it is not, he has the possibility to update his account.
But the documentation deals with "simple" cases and here I am a little lost.
I tested several possibilities (authentication with Guard, test of creation of a personalized provider...) without results.
I would need to understand the mechanisms of symfony authentication to create a custom authentication.
If anyone has a lead to guide me, that would be great.

Thank you in advance.
