Think of the NGINX proxy in Lemmy’s docker-compose.yml file as the entry point to Lemmy from outside the Docker network. For instance, I don’t have any ports mapped for the individual services except for the NGINX service. The NGINX proxy in this docker-compose file will access the other services through the internal docker network, so it isn’t a problem if you set up your nginx.conf file with the service’s names. With that done, you could map any port you want for the NGINX service from the host, then point your internet-facing reverse proxy to that.
I also plan on setting up a Mastodon server, but I haven’t gotten to it yet. So I don’t have anything specific to add other than it will work similarly by using docker’s port mapping or service names depending on whether each service needs to be internet-facing or only communicate internally.