Periodically I get requests from customers to downgrade their iPhones and iPads to an earlier version of iOS. Unfortunately this process can be very difficult and generally it's not possible. The question though is why? Let me describe in layperson's terms the reason for this. While I'm certain there are advanced level programmers out there that will say otherwise, I function from a general user perspective, so my explanation is from that vantage point. I plead no-contest to any of the IT pros out there that might see this. You're better than me for a reason.
One thing I can say about Apple is that they are not a company that takes security lightly. You'll hear some people say that Macs cannot be hacked. That isn't true, but I will say they tend to be less prone to it due to a combination of Apple's closed ecosystem and the actual operating system itself, which is not as popular with hackers as Windows based systems are. But to say they're not vulnerable is inaccurate. Any digital device can be hacked if the right person wants to do it.
In regard to the iPhone and iPad, they too are fairly secure devices. As far as firmware downgrades, here's what you need to know. Every iPhone or iPad out there has a unique "key" to its firmware. In Apple lingo, this key is called "SHSH Blobs". I won't get into the technical details, as it's not relevant for this discussion. Just remember that the device in your hand has a unique identifier, not unlike your car or house key in a manner of speaking. iOS, the operating system on that device, is written so that your "SHSH Blob" will unlock it and allow you to use the device.
The problem with downgrading is that Apple only allows you to do so for very brief periods of time. If you upgrade to the latest iOS and dislike it, they usually give you a few weeks to roll back before they stop "signing" it. Simply put, after Apple stops "signing" the previous version, your "SHSH Blobs" no longer can unlock the firmware. That's more or less how it works in a simplistic way of explaining it.
The question is, how do you work around this? The key, or "SHSH Blob" is the answer. The key on your device changes with every firmware update. So if you're running the latest iOS, which at this time is iOS 16.6, your device's key, or "SHSH Blob" is for this version. If you try to roll back to iOS 8 or something like that? It won't work because your key is for 16.6, not for 8. So in simplistic terms, you need the actual unique key for your specific device for iOS 8. That's why rolling back is a challenge because the key changes with every update and isn't reverse compatible.
The way around this typically is to back up your "SHSH Blobs" before upgrading. There are specific software packages that can do this, but it does take some expertise to do it. Unfortunately, if your current device is on 16.6 now, there's really no easy way to get your key for a previous operating system. I'm sure there are people out there more advanced than I am that might know how to do it, but I personally do not. What it boils down to is that if downgrading is your thing, you have to back up your "SHSH Blobs" for your phone or tablet before updating and keep a cache of them.
That's basically how it works in layperson's terms. Like I mentioned earlier, I have no doubt there are people out there in the IT field who know way more than I do about this, but from a practicality standpoint, this explanation is accurate enough to understand how Apple does things from a big picture perspective.