While integrating a merchant's product information management system with Magento, you create a module MyCompany_MerchantPim that adds a catalog product EAV attribute pim_entity_id programmatically. In which type of setup script do you create the EAV attribute?

A custom module must make changes to the schema following each setup:upgrade run. This must be done after all other module's schema updates have been applied.
How is this accomplished?

You see this code in etc/adminhtml/routes.xml:
<route id="mymodule" frontName="user-subscriptions">
<module name="MyCompany_MyModule" />
</route> You have placed a controller in Controller/Index/Subscribe.php.
If you want to create layout XML instructions for this controller, what would be the layout XML's filename?

You are adding a child node to the product.info block using the XML:

How will this block be rendered?

You are creating a module that will be sold on the Magento Marketplace. You wish for this module to be easily extensible, and decide to add the capability for other developers to utilize extension attributes. What is the minimum update necessary to enable this capability?