You need to trigger the code to run.
That can be done when a Sheet cell is edited, with an installable On Edit trigger.
Or you could run code from a button, or sidebar, or custom menu, and the code could evaluate multiple cells in the same run instance.
Existing Cal. events can be updated either with the built-in CalendarApp service or the Google Calendar API.
So, it's possible to sync from the Sheet to the Calendar event.
The most difficult part might be finding the correct Cal. event that the cell should match to.
You can search by date and time, but it might be better to get an event by the event ID.
You'd need to store the event ID in the cell note when the event is first created.
I searched the Marketplace for add-ons and found:
But don't try to install them from the link given.
From your Sheet click Add-ons, get add-ons, and search for the add-on by name and install it from there.
The G Calendar Sync add-on is higher rated, and the other one might charge something.
And the G Calendar Sync add-on looks like it's open source at:
If you find out anything about those add-ons I'd be interested to know what your impression is of them.
Here's a YouTube video that looks good.