in fact it is in a number of places.
First, under the Apple Menu (Top Left of the screen) "About this Mac" you will find the the first window that pops up includes "Software Update"
Second, the App Store, which is probably toward the left side of the dock and looks like a white letter A on a blue backround
Clicking on that one takes you to the App Store (really?) where you will find a number of options at the top of the window. One is "Updates"
This will show you all the updates that you have installed and will also check for updates when choose the updates icon.
Third, when you get that little notice that you have a software update available, go to the App Store > Updates and you will be able to see what it is.
Hope that helps
What happened to "Software Update," and when I get notification that I need to download an update, why can I not see what it is I am downloading? I must be missing something... even after I have upgraded or whatever, I don't know what I have done. I don't like that. (I have High Sierra 10.13.6)
Old Granny Lola