It was Fairphone OS. I didn’t manually install TWRP, but it also wasn’t installed when I purchased my phone in 2016. I believe it was included in one of the updates.
Also, as of five minutes ago, TWRP no longer loads. I believe I am in bootloader, as the screen just says FAIRPHONE, powered by Android, and the charging light is blue.
Is it possible to send the phone back to Fairphone to be reformatted, do you know? I think I’ve reached the end of my rope here.