Which hook to use to give a user a role after purchasing something in Commerce 2? I made a site that sells services in Commerce 2.  I want to hide access to the "Orders" and "Payment info" pages until a user purchases the first product.

To do this, I created a role, Paid, for users who have purchased something.  This role has permission to view orders and so forth.

Which hook should I use to give the user the Paid role after making a purchase?
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.