> pdfTEX is based on the original TEX sources and Web2c
So it at least ties back to original code. XeTeX appears to be similar but with even more extensions, but other of the "TeX" tools are complete rewrites.
Maybe, but I've moved on to tectonic. Which surely isn't in the same language.
Last I checked (though it's been a while), I think tectonic basically wrapped the xetex code.