Welcome to the forums!
I just tried the search - it’s actually the :search fiuntion that doesn’t work, because there is information there CORE Hardware Guide | TrueNAS Documentation Hub under Storage Solutions > Hybrid Storage & Flash Cache (SLOG/ZIL/L2ARC). There you will find an expanded version of @Fleshmauler’s comments - however, they may be a bit opaque to you as ZFS doesn’t use “cache(s)” in the same way that does e.g.windows, nor does it use the word cache in the function name.
Go here Introduction to ZFS | TrueNAS Community
and download that primer by mod @ericloewe and find his introduction to the “ZFS cache” topics on page 11.
This by @Arwen is additive:
Welcome to TrueNAS forums.
There is no such thing as a ZLOG, it’s SLOG or Log device. It is always helpful to use the proper terminology. ZFS does not have a write cache, the SLOG is not a write cache. All pools have a ZIL for synchronous writes, which is inside the data vDevs.
Further, if you have to ask about SLOG, Special Metadata vDev or L2ARC / Cache devices, you probably don’t need them. They can always be added after.
In general, L2ARC / Cache device(s) would only be 5 to 10 times your RAM size. So with 256GB of RAM, using 2 x 1.92TeraByte SSDs for L2ARC / Cache is a bit too much. This is because the pointers to the data in the L2ARC / Cache are in RAM. Too much L2ARC / Cache and too much RAM is used.
I highly suggest reading about ZFS and it’s pool configuration options. I don’t have a list of Resources or docs to read, but you can find some in the Resources section of this forum. (See left hand side, Resources’ Categories.)
Formerly of Rivendell,
Now far east, near Mordor, Middle Earth
With respect to “what” on storage, start in the docs with the simple at Storage Configuration | TrueNAS Documentation Hubexam where a mirrored storage pool is created.
But quite likely you want a more potentially robust (more redundancy) - go back to Introduction to ZFS, pages 5 and 6 and on for an overview that includes RAIDZ1, RAIDZ2 and RAIDZ3.
By then you may want to come back here with more questions …
When you come back, best if you describe your intended use for your data store and your expectations of its management, particularly backups and recovery.