In Notes app, how do you reverse a delation?

No, it does not. There is no undo, or redo button in notes. Once you have made an edit to a note, it is saved. You could restore a backup of the phone to restore an older note, if you have a backup made when the note was on the iPhone, but that would be the best you could do if you edited a note. That may allow you to bring back your deleted note, if you have not created another backup since you made the deletions.

You might want to have a look at this article, telling you how to use the iPhone´s "Shake to undo" feature:

https://mashable.com/article/how-to-use-shake-to-undo-ios-iphone-ipad/?europe=true

Hope this helps you! Regards, Ian.

If you deleted a note, then go to the Recently Deleted folder and you can recover it from there. If you deleted it from the Recently Deleted folder, then the note is gone. There is no recovery.

You're welcome.