Форум поддержки Thunderbird

Emails with different subjects are getting threaded together. For example, in the attached snapshot, why are the emails with subject "Bid S011090833" getting threaded under "Hein Revised Hall Bath..."?

Perhaps I don't understand how Thunderbird determines messages have a common thread.

Using Thunderbird 115.4.1 (64-bit)

New windows 11 computer. Set up Thunderbird with gmail account and I can send and receive emails. however, when I tried to find the profile folder, I noticed there is no Thunderbird folder in the Roaming folder. I can not find it anywhere.