The indirection that exists in Btrfs is the internal virtual address space and block groups. Metadata and data extents are referenced in virtual addresses, byte

The indirection that exists in Btrfs is the internal virtual address space and block groups. Metadata and data extents are referenced in virtual addresses, bytes. That address is translated by the chunk (block group) tree into two things: a physical device, and location on that device. Conceptually it's not altogether different from ext4 on LVM, and hence the old critique that Btrfs is a layering violation. But these things are integrated on Btrfs, and the chunk is what makes so many things flexible, as well as a PITA when it comes to ENOSPC (mostly solved these days).