The bell on your Substack account is the Activity section.
Here, you'll find likes and replies to comments you've made on Substack, likes and replies on notes you've posted, mentions and restacks in notes, notifications for new subscribers and followers, and recommended publications that writers you subscribe to make.
You'll also be able to like, reply, restack, or share a note straight from the Activity Tab.
No one but you can see the items in the Activity section but the content that appears there can be things that have happened publicly on a publication.
Note: Your Activity section cannot be cleared nor notifications disabled.