The entire point of agile is that a PM can't do that. Large estimates force a PM to go back to stakeholders and revise the desired features to something that engineers have consensus that should be deliverable within the next sprint given their historical velocity.
If you're not doing that, then you're not even in any conversation about agile/scrum.
By saying 8pts the team is effectively saying it's probably a 2 sprint job and individual tickets are not supposed to be 2 sprints long meaning there's complexity that either needs to be broken apart or on the rarer case, team agrees the ticket is fine as-is then it'll take 2 sprints.
Anything 13 or greater is saying this ticket is so big we can't even begin to accurately estimate it - PM take it away from us! It's fun to see 21 or 34 pts... it's all a joke at that point.
They're supposed to represent a measure of complexity.
How much time that then takes depends on who does the work, the order in which tickets are implemented, how much distraction there is, luck (do you encounter any unforeseen problems), and so on.
They're then used to see how many issues will fit in a sprint, which is a measure of time. So everybody ends up treating them as representing time anyway...
But in reality it's ideal hours or ideal days.
The 8 and 13 come from the fibonacci sequence.
> I have never seen feedback from estimation to requirements.
Perhaps you work with perfect pms! jokes aside, usually all it takes is an estimation of 8 or 13 and something changes real fast.