“Skipped the docker bit”? I’m assuming you mean by using the proxmox helper scripts? If so, the path to the config file is there as well. If you are setting this all up on your own without the helper script, then the config path is whatever you choose in your compose.yaml.
Frigate also has a built in config editor, so as long as you have it up and running with the correct storage path you can edit the configuration on the front end.
TBH, the phrase “skipped the docker” is a bit confusing. Running in LXC doesn’t mean you are not using docker, the script installs docker on the container. I personally don’t run it in a container, passing through storage and detector hardware can be challenging. Just spin up a small Debian VM, install docker and compose, and follow the frigate documentation. It’s very well written, and almost every problem you would run into is covered. It will give you the basics of the inner workings, and makes it much easier to pass through hardware and storage.