Code for configuring custom user builder for Azure AD integration with sitecore

CustomExternalUserBuilder.cs

public class CustomExternalUserBuilder : DefaultExternalUserBuilder
    {
        public CustomExternalUserBuilder(string isPersistentUser) : base(bool.Parse(isPersistentUser)) { }

        protected override string CreateUniqueUserName(UserManager userManager, ExternalLoginInfo externalLoginInfo)
        {
            Assert.ArgumentNotNull((object)userManager, nameof(userManager));
            Assert.ArgumentNotNull((object)externalLoginInfo, nameof(externalLoginInfo));

            identityProvider = this.FederatedAuthenticationConfiguration.GetIdentityProvider(externalLoginInfo.ExternalIdentity);
            string defaultDomain = "sitecore\\";


            if (string.IsNullOrWhiteSpace(externalLoginInfo.Email))
            {
                if (identityProvider != null)
                {
                    defaultDomain = identityProvider.Domain;
                    var emailClaim = externalLoginInfo.ExternalIdentity.Claims.FirstOrDefault(x => x.Type == "email");

                    if (emailClaim != null) return defaultDomain + emailClaim.Value;
                }
                
                var validUserName = externalLoginInfo.DefaultUserName.Replace(",", "").Replace(".", "").Replace("'", "");

                return defaultDomain + validUserName.Replace(" ", "");
            }
            return defaultDomain + externalLoginInfo.Email;
        }
    }

CustomExternalUserBuilderPatchConfig.config

<sitecore>
    <federatedAuthentication>
        <identityProvidersPerSites>
            <mapEntry name="all sites" type="Sitecore.Owin.Authentication.Collections.IdentityProvidersPerSitesMapEntry, Sitecore.Owin.Authentication" resolve="true">
                <sites hint="list">
                    <site>regexp:.*</site>
                </sites>
                <externalUserBuilder type="BasicCompany.Foundation.Extensions.UserBuilders.CustomExternalUserBuilder, BasicCompany.Foundation.Extensions" resolve="true">
                    <IsPersistentUser>true</IsPersistentUser>
                </externalUserBuilder>
            </mapEntry>
        </identityProvidersPerSites>
    </federatedAuthentication>
</sitecore>