Thank you for your good will and promising ideas. Yes, at least two other devices of mine are syncing to the same Apple ID, and yes, some file sharing is supposed to happen. You've given me a new direction for my troubleshooting.
Meanwhile, since my posting, I have contacted Apple support directly and with their help have freed up over 100 GB on that drive. The crisis of low space has thus been fixed, at least temporarily.
But the /Keychains/ file on the drive continues to grow as something keeps trying to logon to the O.S. Small files, each the same size, about 350 MB, are being added, each with the name login.keychain-db.sb-????????-?????? .
It's like a tumor growing on my internal drive!
Will be grateful for further help.