This hint was given by a comment by user1340531:
Mine is at C:\Users\{username}\AppData\Local\Microsoft\Windows Terminal\settings.json
(or more generically: %userprofile%\AppData\Local\Microsoft\Windows Terminal\settings.json
)
If it's not there, you should consider OP's answer or vyps comment which lets you find out one of these generic paths (they are equivalent):
%userprofile%\AppData\Local\Packages\Microsoft.WindowsTerminal_8wekyb3d8bbwe\LocalState
%LocalAppData%\Packages\Microsoft.WindowsTerminal_8wekyb3d8bbwe\LocalState
$env:LocalAppData\Packages\Microsoft.WindowsTerminal_8wekyb3d8bbwe\LocalState
If you don't know what these paths mean, I'd recommend playing around with "Windows Run" (Win+R) and the explorer address bar.