For instruction or help text, I think it should be before the field. This hopefully reduces the chance of error. Introducing a “Do it this way” after you’ve done it makes the user feel less powerful.
For validation, after makes sense. Here are the rules, do the thing, I did the thing right or wrong is a reasonable flow. But do this thing, do the thing, here’s how to do the thing doesn’t make as much sense. Yes, we should all know emails need @. But did you properly code you phone number field to accept 1234567890, (123) 456–7890, 123.456.7890?