Connecting iPhone 12 with iOS 18 to Mac running Sierra 10.12.6

I could be mistaken, but I believe that all Macs that can run Sierra can upgrade at least as far as High Sierra.

How to download and install macOS - Apple Support

If you upgrade, I would strongly recommend making a bootable clone backup of your system (using Carbon Copy Cloner or SuperDuper!) first.

Use the How to download and install macOS - Apple Support article to which I already posted a link.

There is a High Sierra link in it that will cause the App Store to show you the normally-hidden page for the High Sierra installer. This link will only work on a Mac that is compatible with High Sierra. You may need to be using Safari, and have Safari set as your default browser, to get the link to work properly. (I remember having trouble with Firefox and being told that the item I requested wasn't in the App Store. There seems to be some "secret" handshake going on between Safari and the App Store that other browsers might not know about.)

I don't think that Macs running Sierra can manage iPhones running iOS 18.

The iPhone 16 was released at the same time as iOS 18. If you look at the Technical Specifications for the iPhone 16, you find that

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Syncing to a Mac or PC requires:

• macOS Catalina 10.15 or later using the Finder
• macOS High Sierra 10.13 through macOS Mojave 10.14.6 using iTunes 12.8 or later
• Windows 10 or later using iTunes 12.12.10 or later (free download from apple.com/itunes/download)

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I believe that applies generally to all iPhones running iOS 18, even if you could manage some of them using a Mac running El Capitan or Sierra when they were running a lower version of iOS.

If iTunes 12.8.3, running under High Sierra, works properly, you will use it in the same fashion as you have been using iTunes under Sierra.

That is an “if” - because you will find a number of reports in these forums of people who had no success managing iPhones running iOS 17 using Macs running High Sierra. But if you stick with Sierra, you will be using a configuration that isn’t even “supposed to work” any more.

Once you're on High Sierra, check what version of iTunes you have.

If it's lower than 12.8.3, you can find iTunes 12.8.3 for Mac here:

https://support.apple.com/en-us/docs/software/pl295