A collection URL like /users accepts GET to list every item and POST to create a new one. At creation time the new user doesn't exist yet, so it has no id — the client sends POST to /users, and the server picks the id when it creates the user and returns it in the response.

An item URL like /users/42 accepts GET to read that one item, PUT or PATCH to update it, and DELETE to remove it. POST on an item URL and PUT or DELETE on a collection URL aren't part of this mapping — /users has no id saying which single item to replace or delete, so PUT and DELETE there have no clear target.