Large genp.bt3 file taking up space (Mojave)

Thanks for your response - and apologies for not explaining it clearly to start with. When I drag the subfolder he talk about in his post from a few years back into the Bin and empty it, the Keychains folder immediately recreates that subfolder - making it virtually impossible to remove it in line with those instructions.

From all the research I have done, it seems like it is related to iCloud Keychain but I can't tell what it is - genp.bt3 is a Document file but when I try to open it, I get a message saying 'there's no application set to open this document.;

Thanks so much for your time!

I actually had a chance to drop into an Apple store yesterday - and the lovely man there was puzzled by the whole thing just as much as I was. After that, he suggested for the file to be deleted (firstly, he unticked Keychain in iCloud settings) - and voila, cleaned 50GB and the computer/ system is working fine.

Doesn't necessarily help crack what the problem is - but ultimately it helped me solved the main problem which was reclaiming some space on disk.