As has been pointed out above, when your computer is properly shut down nothing will be running.
If Screen Time is showing that Finder has been running for 18 hours, then one might assume that Screen Time is misreporting the Finder uptime, or that the computer is not being properly shut down.
Regardless of which is accurate, it really does not matter one way or the other what Screen Time reports. This is simply a way to monitor what apps are running and for how long. It has no effect on the useful life of the machine. The perceived discrepancy of actual and reported runtime is odd, but I wouldn't put much weight in this. Just know that any time the Mac is running, so is Finder. It is thus for all Mac computers.
Do you have more than one Mac in your home?
I ask because there is a setting in Screen Time setup that allows for the reporting of combined application times across more than one device. Look for "Share across devices" on the Screen Time setting panel.