calendar-app | Simple calendar application to keep a note of appointments | Calendar library

I have a sheets-bound script at the moment that reads the spreadsheet data and creates calendar events based on the sheet. That spreadsheet also stores other user's calendarIDs (not owned by me) for which I am trying to read/write their google calendar's.

My ideal usage flow was that a single user could trigger the script for all stored users OR each user could click on the script and have their events added to their specified calendar but I found that I am unable to even access other's calendars with just their calendarIDs.

My code was crashing earlier after I created a test calendar on a different google account. I toggled the calendar to public and am now at least getting null as indicated by the getEvents() documentation. I then subscribed to that calendar and received "exception: Action not allowed".

I have come across OAuth2 and feel like this is something I need to somehow implement but I was not sure how to go about it. Am I able to make changes to multiple usere's calendars from the script or do I need to learn about webapps and would that allow me to somehow access other's calendars?

Edit: I'm not a domain user, just using private personal google accounts

Edit 2: Also found out add-ons are a thing. Going to read about that

When I set S3 Bucket PolicyI suffered Access Deniederrors.

My json file is like below. I couldn't figure out why such kind of errors occured. If someone has some experience,please let me know.