n8n does not store SSL certs anywhere. Traefik is taking care of all of that. It will save them in a subfolder of ${DATA_FOLDER}/letsencrypt. It should normally actually also auto-renew that certs before they expire.
Yes, you can use what you wrote above to use custom certs.
If you’re using this in a docker container in a server most likely there are other applications running in it. It would be great for us to get a simple guide helping out on how to use custom certificates in a installation.