how can I get a new Mac with an old operating system

You can't. If you try to run a new computer with an old OS it may well crash since the old OS won't recognize all the hardware in the new computer. The best you can do is to try and run a virtual machine alongside the new OS.

Sorry, yes, I saw that. I was writing that more for the AR person (who, or course, will never read this) because I am seeing frequent cases where they don't seem to fully analyze a scenario and its fine aspects before marking something. :-/ The AR person simply went with the reply saying you can't install it, not thinking it fully through to the extent that you could, for example, try booting to a second drive with the OS installed on that from another computer and not even need to install it on your main drive. Not that it will necessarily work, but that's where I think the reply I posted (and others also noted) more closely describes the actual problem and reason for not trying to boot to an old OS, not just that it isn't possible to install. I knew it wouldn't install, I was just thinking several steps ahead (including some I have not mentioned because Apple might not like it), it won't do you any good and that's the real reason for not trying.

As stated, it's not possible. You can't even install an older version of macOS than what comes installed with a new Mac.

if the os is older than the mac then it has no drivers for the mac hardware

you may be able to run older macOS versions on the mac using a virtual machine like parallels though

Why do you think you need to do that?

Maybe if you provide more details, we might be able to suggest an alternative.

There are ways to try to boot to an older OS without installing it on the computer, which is why I phrased my response the way I did.

I don't understand your reply. I agreed with you.