Apple are quite ruthless with making their old tech obsolete and unsupported.
I've even had trouble getting the SAME operating system back onto a Mac that is old. https://www.ihatemypc.com.au/post/ubuntu-for-your-mac-too-when-apple-leaves-you-high-dry
Most IT and technology implements some form of planned obsolescence but Apple have refined it to a very annoying art.
If you have a Mac/iMac/Macbook/Mac-Mini and it is more than about 5 years old you'll find you won't be offered the latest operating system to update to.
This recently happened to me.
My iMac is going like a train now that I upgraded it with a Solid State Drive so to be denied an upgrade to Big Sur (the latest MacOS available at the time) really annoyed me.
So I looked up how to force it onto my Mac and see if in fact that was a good idea.
NOTE: Now there's a newer MacOS again, so I'll probably be trying this again soon with Monterey.
However, after searching for the best way to do this I landed on Patched Sur, a primarily automated process that did a really good job for me.
These tools are created by individuals or groups of individuals with more time on their hands, and better skills than me, and they are best used some time AFTER the upgrade initially becomes available.
A lot of trial and error is involved and if you let the folks who can handle the crashes and failures deal with them first then subsequent versions of the tools will work more smoothly for people like me/us who don't want the hassle...who just want the latest Operating System as easily and painlessly as possible.
Having said that, this isn't a set and forget process.
You do still need to make sure that your machine meets the requirements outlined by the "Patched" process.
For example, in my case, certain types of graphics adapters meant that regardless of being forced on, Big Sur may still not work.
Luckily my iMac was "compatible" so I pressed on with good results.
It isn't a quick process either as the Patching uses tricks that require the full OS be downloaded (8GB or so at a time) and then, from that point on, all updates will be managed by the Patcher and require the full 8GB download each time.
So decent internet is a pre-requisite (Australia I am talking to you).
So while the variables are many, the point is you don't have to just sit back and put up with Apple's crap.
You do need to be a bit techie and ready for failure (backups people, backups) before you try it.
So good luck and have fun with that.