If I understand the Question correctly, you need to use the XML_SCHEMA_NAMESPACE built-in function to extract the Schema Collection, and you can find the list of those in the sys.xml_schema_collections system catalog view. You can use something along the lines of:
SELECT CONVERT(NVARCHAR(MAX), XML_SCHEMA_NAMESPACE(sch.[name], xsc.[name])) AS [XSD]
FROM sys.xml_schema_collections xsc
INNER JOIN sys.schemas sch
ON sch.[schema_id] = xsc.[schema_id]
WHERE xsc.[xml_collection_id] <> 1;
I put the WHERE xsc.[xml_collection_id] <> 1
filter in there to avoid getting the following error:
Msg 6314, Level 16, State 1, Line 1
Collection specified does not exist in metadata : 'sys'