How do we typecast or convert a variable of the type sys.XMLType to NUMBER and VARCHAR2?
Your question is too vague to give a relevant answer.
Typecasting doesn't make sense in this situation, XMLType is an opaque datatype, it's not like converting VARCHAR2 to NUMBER (or vice versa).
Do you mean serializing the XML document, or extracting atomic node values, or something else ?
Depending on your requirement, you may want to look for XMLSerialize() or XMLCast() functions.
And on a side note, don't always expect people to search for your previous threads to find some useful information, such as a db version...