Which hook to use to give a user a role after purchasing something in Commerce 2?
In Drupal 8 Commerce is using Event dispatcher instead of hooks.

In this case, you should subscribe into the event named commerce_order.order.assign, the hook invoked then a order is assigned to a user. In this moment you can code to grant the role to the user. You can see an example of how commerce_log submodule from commerce used this event here.

If you need, you have this tutorial which tells you how to subscribe to a event from event dispatcher.