for res in xml.xpath('/gmd:MD_Metadata/gmd:distributionInfo/gmd:MD_Distribution/gmd:transferOptions/gmd:MD_DigitalTransferOptions/gmd:onLine/gmd:CI_OnlineResource', namespaces=xml.nsmap):
print(res.xpath('gmd:protocol/gco:CharacterString/text()', namespaces=xml.nsmap), res.xpath('gmd:linkage/gmd:URL/text()', namespaces=xml.nsmap))
For the Catasto (at least) the url should be taken from the linked XML.
Links are in:
There is also a field containing "protocol":