Get the category id whenever user checks any category in Post Edit Screen

Hello,

I am trying to add separate metabox to assign the primary category whenever users selects multiple category. But I am not being able to get the category ID whenever user checks or unchecks the categories? All the category checkbox contains the value as 1. I am not sure how WordPress is getting the category ID from checked categories.

Reference screenshot:
https://prnt.sc/7FxrIvvswuu_

Any help will be highly appreciated.

Thanks.