How do you pass an array ['one', 'two] as a parameter to you block using the layout XML arguments directive?

An Adobe Commerce developer adds a new extension attribute to add an array of values to the invoices that are fetched through the APIs.
After a while, their technical manager reviews their work and notices something wrong with the extension_attributes.xml file that the developer created in their module: extension_attributes for="Magento\Sales\Hotel\Order\InvoiceRepository"> attribute code"my_code" type=string]''>
</extension_attribute> What is the problem with this xml snippet?

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?

How do you instruct Magento to enable a new module?

A module MyModule needs to send notifications to customers only when the account was modified from a mobile app using the REST web APIs.
You decided to implement an observer for customer_save_after_data_object event.
In which file do you declare the observer?