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

What will be the effect of this declaration?

You are tasked with ensuring customers who log into the site are authorized. By default, this consists of ensuring the customers email and password match the values in the database. On this project, you need to verify additional data in this process.
Keeping in mind upgradeability, how is this done?

You are adding a new menu item to the admin backend which will link to a custom backend page.
The declaration of the route:

What do you specify as the menu item action attribute value so it links to /admin/mycompany/mymodule/?

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

What will be the effect of this declaration?

In the module located at app/code/MyCompany/MyModule there is a JS module in the file view/frontend/web/register.js. The Magento base URL is https://magento.host/ and the luma theme with the en_US locate is used.
What is the public URL for this file?