When creating a jail, an entire Linux filesystem is created in the 'rootfs' folder within the jail's folder of the jailmaker directory E.g /mnt/tank/vault/jailmaker/jails/jailname/rootfs. No files from the TrueNAS host will be available.
Common locations for services are:
/home for user accessible files
/var/www/ for webpages
/tmp for temporary application data such as build files
To allow file access by either the jail, another jail, or TrueNAS a bind can be made. A bind creates a link between two locations. Think of this as a portal, anything that goes in one side is visible from the other side and vice versa.
Note that creating a file in the jail or TrueNAS will reflect in both binded locations, so be careful of overwrites and corruption.
Add the following to your user arguments during setup or into the jail's config file, with your two linked locations separated by a colon:
--bind='/host/path/to/:/jail/path/to'
Where /host/path/to/ is the folder on the TrueNAS filesystem you want shared.
And where /jail/path/to/ is the folder you want those shared files accessible by the jail.
A use of this is making files available in a jail for it to use or serve, such as media files in Plex/Jellyfin:
Example: --bind='/mnt/tank/content/:/media' will make any files inside the content dataset of the tank pool available inside the jail's /media folder. To visualise or test this you can copy some files to /mnt/tank/content/ such as media1.mp4, media2.mkv and photo.jpg. Then change directory to that folder inside the jail cd /media and list files in that directory ls -l where those files should appear.
Do not bind your TrueNAS system directories (/root /mnt /dev /bin /etc /home /var /usr or anything else in the root directory) to your jail as this can cause TrueNAS to lose permissions and render your TrueNAS system unusable.
Best practice is to create a dataset in a pool which also allows zfs, raidz, permissions, and backups to function. E.g creating a websites dataset in a pool named tank then binding --bind='/mnt/tank/websites/websitename/:/var/www/websitename/'