A "username" in SAML can come from the IdP in any form. It is contained in an attribute which you either know beforehand or you ask the IdP maintainer to release for you. Have a look at the SAML
In the response there is an
AttributeStatement containing multiple
Attribute assertions. One of those
Attribute assertions is
uid which is interpreted as meaning "username". In this case the "username" is "test".
EntityDescriptor is part of the SAML2 Metadata specification and is not used to hold attributes. It's used to describe entities such as SP and IdP so they can trust each other.
For deciding which parts of an application a user should get access to you can use the
Attribute. This usually contains a URI or URN such as