I have purchased the Essentials online package and would like to know the best phone App to go with this, and is it part of the package?

You do not necessarily need an app to use your browser based product on your phone, in fact you may not like the shortcomings of even the official QBO app from Intuit (you cannot do 100% of what you can do from a full desktop or laptop with full sized browser) but graphics and text within the official QB app (available in the app store for your phone. Android or apple) will be better suited to the small screen

There are many third party apps for specific functions available as a companion or plugin to QBO, depends on your needs, receiving payments, managing CRM, logging miles.