How to Implement Comments/Like Notifications

What you're asking is notifications, which you'll eventually save to DB.

With that implementation, this alert becomes a list of alerts that require users to refresh themselves. If you want to inform the user of the notification INSERT incident without the user's refresh, it is a different topic called event broadcasting, so please refer to Jung Young-hoon's comment roughly.

There is no table in mongodb, but please understand by filtering it moderately.