what MacOS software needs updating so I can connect to iPhone?

I just connected my iPhone to my MacBook Pro (15-inch, 2016) and get the message that "A software update is required to connect to [phone name]." It doesn't indicate what software needs updating, nor does it indicate whether a restart will be required.

I am running MacOS 10.13.6 (High Sierra). I absolutely do NOT want to upgrade to Mohave as I have a full web backend development stack and don't have time to reinstall everything right now.

So I clicked the "Learn more..." button which goes to https://discussions.apple.com/thread/250058909 and it just restates the obvious, which is completely unhelpful.

I searched the forum and found this: https://discussions.apple.com/thread/250058909. There is a post that claims to answer the question, saying "In this case, it will update macOS to the current version that will work with your Mac." What does this mean? I think of updates as minor, whereas going to 10.14 would be a major upgrade. But I'm running the latest 10.13.x.

If no one knows what it wants to update/upgrade me to, can you tell me what will happen if I click the "Install" button? Will it just start overwriting my OS, or will I have a chance to review what it intends to do?

This is so frustrating, especially as it upgraded my phone in the middle of the night without asking my permission. Why do they keep breaking things that were working just fine?

Thanks for any help!

Donna