I believed I used the terminal, or perhaps the software manager, to install the files from repos. This is further supported by the fact that I don't have a copy of the milestone appimage lying around. I don't think this counts as building it myself though.
I don't seem to have any openxcom.log files. I did find that I still have an openxcom folder containing the UFO data folder in /usr/share. I have the UFO game files in a folder on my desktop, in which I also put the nightly appimage. The instructions on the wiki & linked from there all make sense to me except that I
don't didn't know where the listed folders (user, data, config) are. I finally understood what the paths on
the Git mean and found them, including the log file showing the same locations I found them in. I never realized that OXCom made these directories upon installation. I've restructured the dirs according to
this separate UFOpaedia page. I still don't have any of the files/folders listed in standard font (as opposed to bold) but the game's running now.
Thanks for your assistance. I'd seen the video before but was daunted by its length, only just finding out that there are links to the relevant part of the video. In any case, if I haven't bounced my issue against you, I'd have just given up. And now to suffer at the hands of alien invaders.