A user is default synchronized so the name is taken from ClaimsIdentity.Name (if you reflect that implemenation then you can see that it reads the claim with type http://schemas.xmlsoap.org/ws/2005/05/identity/claims/name)

The other claims that is automatically synched (if they are present) are specified by EPiServer.Security.ClaimTypeOptions, which looks like:

/// <summary>
    /// Defines claim types that are synchronized to cms for a user.
    /// </summary>
    /// <remarks>
    /// By default is claim types defined in <see cref="ClaimTypes"/> used.
    /// </remarks>
    [Options]
    public class ClaimTypeOptions
    {
        /// <summary>
        /// The claim type for email
        /// </summary>
        /// <remarks>
        /// Default value is <see cref="ClaimTypes.Email"/>
        /// </remarks>
        public string Email { get; set; } = ClaimTypes.Email;

        /// <summary>
        /// The claim type for GivenName
        /// </summary>
        /// <remarks>
        /// Default value is <see cref="ClaimTypes.GivenName"/>
        /// </remarks>
        public string GivenName { get; set; } = ClaimTypes.GivenName;

        /// <summary>
        /// The claim type for Surname
        /// </summary>
        /// <remarks>
        /// Default value is <see cref="ClaimTypes.Surname"/>
        /// </remarks>
        public string Surname { get; set; } = ClaimTypes.Surname;
    }