How do you instruct Magento to enable a new module?

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

What will be the effect of this declaration?

\Magento\Sales\Model\Api\OrderRepositoryInterface::getList accepts a SearchCriteriaInterface to filter and sort information.
What class assists in creating an instance for SearchCriteriaInterface?

Assume that $collection is a new instance of a class that extends
Magento\Framework\Model\ResourceModel\Db\Collection\AbstractCollection, and $ids is an array of ids.
How do you select a list of records from the database where the record ids are in the $ids list?

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?