Is a hash code automatically created for every user file (e.g., document, photo, audio, video) the first time it gets written to the NAS? Or do you have to use some kind of app or NAS utility and enable it to generate and assign a hash code to every one of your files?

And where are those codes stored? Inside of the file’s own container? Or are all user file hash codes stored someplace else? In a “hash table” and/or on a drive partition on the RAID drive array?

Are these hash codes used by the zfs and btrfs file system for routine data scrubbing?

Then, as mentioned in the above links, following data scrubbing, are these hash codes also usually used for routine RAID scrubbing?

But for both data and RAID scrubbing, is data integrity ensured by comparing the hash code of each file with its initially (first ever created) hash code (stored wherever) to the hash code currently in the file. If the system’s comparing calculations show that the codes are different, then one or more of the file’s bits have flipped, so then it knows that the file is therefore corrupt?

If yes, then at that point will it flag me and ask if it wants the system to attempt to repair it?

Thus, for backups from main NAS to backup NAS, how much more is the SHA256 hash function (algorithm) worth using to prevent collisions and to verify data integrity of user files via check summing than MD5, because it uses twice the number of bits?

But if not much more advantageous for even potentially large audio files https://www.hdtracks.com/ , then would SHA256 be a lot more so than MD5 for check summing during for backups of DVD movie rips saved to uncompressed MKV and/or ISO files, because video bandwidths are so much bigger than audio?

But if the app returns a check sum error between the file on my main NAS and the copy to be updated on my backup NAS, how then to repair the corrupt file?

Again, by using the file’s original hash code (stored some place) created the first time that it was ever stored in the NAS?

Is a hash code automatically created for every user file (e.g., document, photo, audio, video) the first time it gets written to the NAS? Or do you have to use some kind of app or NAS utility and enable it to generate and assign a hash code to every one of your files?

what else can I and the zfs and/or btrfs do to both bit rot?

And to minimize the risk crashes:

Replace the RAIDed HDD array every 3 (consumer) to 5 (enterprise grade) years.

And where are those codes stored? Inside of the file’s own container? Or are all user file hash codes stored someplace else? In a “hash table” and/or on a drive partition on the RAID drive array?
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But as hash functions are never perfect, and while rare, data “collisions” are inevitable.

What do you mean by "the file’s own container"? Whatever storage space you're using to hold your files, some of it will be set aside to hold metadata. Metadata includes the hashes as well as things like file names and modification times.

You seem to have a misunderstanding: RAID is for protecting against disk failure, not data corruption.  Data corruption is extremely rare, because of the defences built into the HDD itself and the way the file system exploits that.  For example (and this is what happens on any HDD made in the last 30 years or so): when the HDD reads a sector of data off the platter, it is compared with the CRC (stored with the data) and if it disagrees the read is retried multiple times to obtain (if possible) a valid CRC.  Chances are this will succeed, because a HDD is essentially analogue and if a sector is going a bit marginal it won't be actually dead.  Marginal is good – the HDD's built-in controller can then swap that sector out (invisible to the user) for a sector in the pool of spares.

The file system (not RAID, although the file system may be stored on a RAID) is responsible for ensuring further data integrity, and if a particular file system uses enhanced methods for data verification then the check data (eg hashes) are stored by the file system in its directory tables (which are also subject to rare data corruption).

Far less rare than actual data corruption is file system inconsistency, where the file system indexing loses track (typically because of being turned off mid-update).  Journalling file systems such as BTRFS are designed to prevent this kind of corruption by keeping a log of pending update operations so that they can be re-run if necessary.
 
And, just as a reminder (I wouldn't mention this again if you hadn't started a new thread): RAID management etc etc etc is a HUGH OVERHEAD.  It would be far easier just to have good backups, and if one of those backups were on a subscription cloud service you've got the best of both worlds (and probably cheaper).

"Container" being the file's FLAC, WAV, MKV, ISO, et al.

And how safe would going with them be if my only internet connection is a wi-fi hotspot via my iPhone?

As for as installing and maintaining in-home hardware for data storage and prevention against loss and corruption, rather than adding a second in-home RAID array NAS box to use for backing up the main NAS, you seem to you suggest only using a cloud service for backup.

If yes, and assuming their hardware and software configurations and routines are much better than what I can do on my own to store and protect my data, which would you say are among the top five most reliable cloud service brands?

And how safe would going with them be if my only internet connection is a wi-fi hotspot via my iPhone?

  I don't mean to pry, but as you are so concerned about your data that you are contemplating/willing to pay for subcontracted NAS building and the overheads of running a NAS including pre-emptive HDD swapping, is it a false economy not to have broadband?.........

All cloud storage services offer far greater levels of data security than can be conveniently managed at home, it's just a question of who you think most deserves your dollars.