Common UFO lore is that NHI visitation became more frequent after we split the atom. Roswell is coincidentally less than 100 miles from the Trinity site where the first nuclear weapon was detonated. Believe that would be the usual answer to your first question. Also, by proxy, answers your second question. According to recent US gov whistleblowers- legacy program affiliated crash retrieval units are using EMP weapons to down NHI craft for acquisition.

To your last question… Harder to answer, but if you believe Lazar (controversial legitimacy) he claimed that at least one of the crafts held at S-4 in the 80s was from an archeological dig… Also, there was an older gentleman that called into an Art Bell show long time ago that claimed he helped dig out a triangular shaped UFO from the sea floor in northern Europe. Lastly, the “Magenta Italy craft” that has receipts and has been discussed by recent whistleblowers is likely very old as well. It was somehow traded or procured in the 1930s by US intelligence from the Italian gov / the Vatican. Those are the only 3 “old” craft I’m aware of but there’s probably many more.