On the drive your hierarchy is kept as it is if you use the external library feature. In this realease you can even browse the folder structure on mobile.

In Immich I did have to reorder all photos to their own albums, although I think there is a function that does this based on the folder structure. I did not try this however.

If you have an existing photo library somewhere on a drive, and load it in immich keeping its own folder structure. In the docker compose file you map the external library to the container.