Docker compose files (and docker execs) use the following format

  volumes:
    - <host_storage_path>:<vm_mount_path>

If the docker-compose has a ${VAR_NAME}:/data, then it will replace it directly with the variable defined in the .env file in the same directory with the format

VAR_NAME=/path/on/host

And would mount the host’s /path/on/host to /data

I've never used Linkwarden, but the /data folder is often used by Docker containers to store the application's data, so it's likely an internal path. You'll have to create a volume that exposes the internal /data path to the host filesystem, then whatever is written into that directory will be made available to both the container and the host system. Any file or directory in the container can be exposed this way.

I usually put my data volumes in /srv (where my large RAID array is mounted) and config volumes in /config, into a subdirectory named after the service, and with the minimal necessary privileges to run the container and the service. You could, for example, create volumes like this:

/srv/linkwarden/postgres_data:/var/lib/postgresql/data
/srv/linkwarden/linkwarden_data:/data/data
/srv/linkwarden/meili_data:/meili_data

The volume path (left side of the colon) can be anything. The right side is where the services expect their files to appear inside the container.