You have created a custom module which must perform an action immediately after an order is placed, but only on the store front of the merchant site. You have selected the checkout_submit_all_after as the target event which the module will observe.
In which file will the event observer be declared?

The constructor function for \Magento\Catalog\Model\Category contains this excerpt:

With the automatic dependency injection that Magento provides, how is the StoreManagerInterface resolved?

You added a plugin declaration to MyCompany/MyModule/etc/di.xml:

What will be the effect of this declaration?

In a custom module you are adding a new field to the store configuration. The field will set the value for the configuration path mycompany/mymodule/myoption.
How do you supply the default value for that configuration option?

You are working on a Magento store which will be selling in two countries. Each country has its own set of payment methods.
How do you organize the project to support this requirement?