Check out example codes for "get ad user using email address microsoft graph C#". It will help you in understanding the concepts better.

Code Example 1

// Retrieve a user by id
var user  = await graphClient
    .Users["00000000-0000-0000-0000-000000000000"]
    .Request()
    .GetAsync();

// Retrieve a user by userPrincipalName
var user  = await graphClient
    .Users["[email protected]"]
    .Request()
    .GetAsync();

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