Unintended OS update

I just updated my iPhone 8 while it was connected to my Mac running Mojave and it only updated the iPhone. Like you I hesitated for several minutes to make sure that I fully understood what exactly was going to happen. All went well, in fact it seemed to take less time to upgrade the phone then using the iPhones updater.

However if you are not sure or feel uncomfortably, then it might be best to eject and disconnect, then update the iPhone using the iPhones internal updater.

I got the same ambiguous message, and hesitated also, without knowing exactly what would be updated, and not having time to play around with whatever a major update might break.

I ended up updating the phone directly (not with the Mac), due to an app that already stopped working due to needing the latest phone iOS. After that, I can't sync / backup the phone to the Mac any more either. So I was forced into the macOS update.

The update that downloaded for me only took a couple of minutes, and appears to just be something that has updated within iTunes. It didn't update me to the "new" replacement iTunes, and didn't try to update me to Catalina. So this update looks safe from that perspective - you are likely OK to just proceed and accept the update.

I suspect all that was updated was an internal macOS framework, as mentioned in an older post for the same message here:

https://discussions.apple.com/thread/251462082

If in doubt, I often install suspicious software 1st on the external bootable backup disk. If it is OK, I install also to the main internal disk. If not, I reset the external backup disk by cloning the internal disk to it with Carbon Copy Cloner (I have two such backup disks, the other is reserved for such testing).

It is an update for iTunes, your OS will not be upgraded.