I have looked far and wide over the internet but I still haven't been able to find an answer to what really comprise a SAN Storage node/unit.

I do understand the fundamental difference between a NAS and a SAN, that a NAS is a server which typically serves a folder over NFS or CIFS/SMB while a SAN is a network used for sharing SCSI block devices between SCSI Targets and SCSI Initiators.

There are proprietary solutions sold by various vendors where you seem to buy a hardware product consisting usually of 2 hot-swappable interconnected motherboards (for redundancy) connected to a bunch of disks via SATA/SAS/PCIe on one side and connected to a FC/iSCSI/FCoE-network on the other side. On this machine runs a proprietary OS that takes care of LUNs, access-lists (or whatever they are called), being a SCSI Target and general monitoring of the disks.

These LUNs are then accessed by a host/SCSI Initiator which mounts the LUN as a SCSI-disc onto their system.

Considering my definition above is correct, is a SAN Storage node really just a server presenting storage to a/many host(s)? If so, could one technically create their own SAN Storage node with an old server, a bunch of disks, an HBA-card, and something like LIO or targetcli?

You can use NAS simulator software to do something.

Thanks for the tip, although I am interested in getting down to the root of what is required for a product/solution to be called Storage Node.

I found the following clip from SNIA talking about some storage terminology and common names of things (at time 42:56) where the question came up in which they briefly touched upon that there are things called Storage Nodes and that they are different from other types of nodes and that a way of identify nodes is by them being "The device at the end of the wire".

So with that in mind, does anyone out here know of a more in-depth explanation/specification for what a Storage Node is?

In general, a storage node is typically a physical server with one or more hard-disk drives (HDDs) or solid-state drives (SDDs).