I'm not totally familiar with this in the context of low-level boot.
I've used iSCSI (SCSI protocol over Ethernet), AoE (likewise, ATA) and FreeBSD has a network layer of their GEOM virtual storage stack.
A "file server" takes a local filesystem (like Ext4) and shares it to network clients. Client and server need to agree on the protocol for sharing (SMB for example), but the client need not understand the server's local, on-disk data layout. My Mac doesn't run Ext4.
A Network Block Device just serves up storage blocks, not files. Sort of the inverse of the file server, where the server need not understand the local, on-disk layout of the span of blocks it's sharing. The NBD client interprets the blocks it receives as a file system. I could install an iSCSI driver on my Mac, share a volume from a Linux iSCSI server, and format the volume with APFS. I could encrypt the volume, such that the server couldn't access the decrypted data on its disk. It just serves up "raw" blocks.
NBD is fairly similar to AoE or iSCSI, you can connect to an NBD and then format it, as you suggest doing with iSCSI and APFS. One use case people were doing a lot at one time is doing RAID-1 with a local partition and a remote partition over NBD in HA clusters.
I'm not totally familiar with this in the context of low-level boot.
I've used iSCSI (SCSI protocol over Ethernet), AoE (likewise, ATA) and FreeBSD has a network layer of their GEOM virtual storage stack.
A "file server" takes a local filesystem (like Ext4) and shares it to network clients. Client and server need to agree on the protocol for sharing (SMB for example), but the client need not understand the server's local, on-disk data layout. My Mac doesn't run Ext4.
A Network Block Device just serves up storage blocks, not files. Sort of the inverse of the file server, where the server need not understand the local, on-disk layout of the span of blocks it's sharing. The NBD client interprets the blocks it receives as a file system. I could install an iSCSI driver on my Mac, share a volume from a Linux iSCSI server, and format the volume with APFS. I could encrypt the volume, such that the server couldn't access the decrypted data on its disk. It just serves up "raw" blocks.