If my content server is unRAID, my MC library server is Debian and a client is Win 10, will the client read the content directly from the unRAID SMB share or will the file come through the Debian MC Library box? At least a couple of years ago the answer was it would only read from the content server if the link was identical, IOW, the client system OS must be the same as the Library Server machines OS.
It will get them from the Debian JRiver server rather than directly from the NAS because the file paths won't be the same on the JRiver client and the JRiver server. If the JRiver server and client are both Linux (or both Windows) the client can access the content directly from the NAS so long as the file paths are the same both places. Direct file access cross platform is not currently possible unfortunately due to file path differences between the OS's.
Do you actually want the clients to read directly from SMB? Why? To me this sounds like a bit of a logistical nightmare.
If you just want to send the raw files (without any decoding happening on the server) you can change that in media network settings on the clients.
Direct file access from clients provides significantly better performance/user experience especially with high bitrate video (much better seeking, lower latency, fewer pauses, lower server load, etc.). Streaming through the server (even without transcoding) has overhead and doesn't perform as well as the client just reading a file it can "see" directly on the filesystem whether locally or through SMB/CIFS. Direct client file access also allows for file name based MadVR directives (e.g. setting deint=film for specific shows), which (last I checked) couldn't be set through tag editing.
As an example, my JRiver clients and server are both hardwired using gigabit ethernet to each other and the same NAS. When the clients can access the media directly, Blu Ray mkvs start in about a second, have no problem seeking, and never stop to buffer. When the clients have to stream from the server the films take about three seconds to start, seeking takes a few seconds, and about half the time a film will stop after a minute or two to buffer (usually only once or twice a film, but sometimes more). This is with no transcoding enabled on fast hardware. The difference between the two scenarios isn't enormous on fast equipment with well-behaved networking (although the buffering pauses are irritating), but my experience is that with less capable hardware using wifi the difference between the two can be more significant (i.e. seeking can take a very long time).
The "use local files if available" option in the client settings is designed to facilitate this use case (where direct file access is possible). Sadly it's not possible in the cross-platform context currently TMK.
EDIT: added examples from my experience.