Hopefully, this is helpful to someone in the group: if you are trying to get a visa extension in Bangkok (as I am after entering Thailand on a tourist visa exemption), then you need to go to the 3rd floor of the IT Square Mall (see attached picture). Obviously, this is subject to change. I lost valuable time at Immigration Division 1 before being redirected to this location today.
TLDR : Answer Summary
To extend a visa in Bangkok after entering via a tourist visa exemption, head to the 3rd floor of IT Square Mall to apply for a 30-day extension. Users who entered with a visa exemption should go to specific offices, with notable instructions to file a TM30 form.