Calendar opening on startup and running in the background despite being removed from start-up items on macOS Ventura

I need to start by saying I have been an Apple for everything person since 2006. I have also reviewed all the threads previously posted in this group. (Most current seems to be 2 + years ago)

So, it seems that some legacy data/apps are now causing me some serious issues. iCal opening upon Start-up and even when closed is still running in the background, killing my processing speed. Here's the data:

iMac Retina 5K, 27-inch, 2017 3.5 GHz Quad-Core Intel Core i5

Running: Mac OS Ventura 13.7.8 (22H730)

Internal Memory still available: 1.03 TB

iCal is still opening upon start-up BUT does not show up in start up or Login Items or Allow in the Background lists.

Thanks in advance All!

[Re-Titled by Moderator]

Original Title: I can't get the apple calendar to stop opening on startup OR stop it from staying open in the background all the time