How does migration to /data actually happen?

[Wordsmith9091]

How does the container know to start looking in /data if the user changes their Compose to include a bind mount for it instead of /use/src/app/upload, if that user doesn't/didn't also change the media library environment variable?

I see the default for that variable has changed, but then wouldn't it mean it's ALWAYS looking in /data unless specified, even if a person slips the optional migration and their folder is still at /just/src/app/upload?

Does it actually check both and just waterfall from one to the other?