@makarai That would have been me who said a better understanding of FreeNAS is required, which is true as I'm seeing a lot of posts lately specifically regarding permissions on FreeNAS. This is not anything Emby can control, it is simply how the FreeNAS operating system works.
@edevils You say your issue is not with FreeNAS and it is with Emby, however the issue you describe is 100% in the way FreeNAS is designed to work. You have to remember, FreeNAS is a file system first and foremost with the option to run plugins. It's nothing like Windows or MacOS and was never intended to be a point and click type of system so there's definitely a learning curve with it, and it can be overwhelming while you're still learning it.
When you install Emby on FreeNAS, regardless of whether it is installed via the plugin or the shell, it gets installed into a jail. This is true of every plugin installed on FreeNAS. A jail is in essence a mini system running completely on it's own inside of a larger system. It runs in it's own little world inside the jail with no access to anything outside of it, hence the name jail. This is why you have to mount your files into the jail just so it can see they exist. The problem comes when you have a program (Emby) running inside that jail that is trying to read these files. This same problem will occur with Sonarr, Radarr, Lidarr, SabNZBD, Transmission, Plex....
When you first created your dataset in FreeNAS, you would have created a user and a group that the file system is assigned to, lets for fun say you named them both edevils. Because all of these plugins run inside their own little mini system (jail), the plugins are completely oblivious to what you chose as your user/group for your files. When the jail is created and the plugin installed, the plugin is given a user and group of it's own. In this case, it's emby and emby. If your user or group that you initially chose don't match exactly with what the plugin runs, the plugin cannot read/write/execute your files. This is why you need to go through that step of setting permissions for your files. Again, this is completely on the FreeNAS side of things and is completely different for every single user because each user chooses their own user/group for their file system. You also have to do this exact same step with Sonarr, Radarr, Lidarr, etc.. etc.. etc.
Over on the FreeNAS forums, they do have a thread dedicated to giving plugins permission to your files. To complicate matters more, there are multiple ways to do this and that all comes down to user preference. For example, Makarai uses user permission, I use group permission. So a guide written by him would look completely different from a guide written by me. You could even set your dataset to 0777 which gives everybody permission to do everything (not recommended of course since many of these jails will be exposed to the internet). Here's that thread on permissions
Once you grasp the concept of FreeNAS permissions in jails, you'll be much better prepared to tackle setting up other jails with other plugins. But unfortunately jail permissions are not a "one size fits all" solution.
Now with all of that said, I see you went back to Windows 10 as an OS. There's an Emby server for Windows as well that's still a million times better than Plex will ever be Don't give up on FreeNAS completely though, once you figure out the basics it really does make a great system. You can even create a virtual machine to run Windows in FreeNAS