When Adobe developed XMP way back before the turn of the century, RDF was still in not fully out of womb. But we saw great potential and based the XMP platform on a simplified version of RDF.
RDF is the building block for the semantic web as envisioned by Sir Tim Berners-Lee. It basically is a way to describe the world in simple three part sentences similar to subject-verb-object. XMP simplified this further since it is describing the file itself – so we removed the object – loosely speaking since it was inferred.
XMP is mostly embedded in the files that it describes, there are a few cases such as with Camera Raw where we cannot embed the metadata since the file format does not allow it, so it sits as a “sidecar” (as in motorcycle sidecar) file.