What is a valid use case for an around plugin?

You have created a new block and will be adding this block on every page. The block contains user-specific information and cannot be cached. The block is added to the default.xml with:

What does this accomplish?

You are facing a bug, which is supposedly caused by the customization of
\Magento\Catalog\Api\ProductRepositoryInterface::save().
To resolve the issue, you decide to find all logic which customizes this method. Which two places do you search for customization declarations? (Choose 2)

You are implementing a custom module MyModule, which provides an implementation of
\Psr\Log\LoggerInterface called \MyCompany\MyModule\Logger.
The LoggerInterface has the default preference declared in app/etc/di.xml.
Keeping upgradability in mind, how do you make \MyCompany\MyModule\Logger the default implementation of the LoggerInterface globally?

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?