I found a wonderfully helpful message asked by GoatHater and answered by Dmitry_Fedyuk. which showed how to migrate an old style Mage::app call to Magento 2, namely
Mage::app()->getStore()->getName();
becomes:
$om = \Magento\Framework\App\ObjectManager::getInstance();
$manager = $om->get('Magento\Store\Model\StoreManagerInterface');
$store = $manager->getStore(null)->getName();
My question, which I think could help many developers, is: "where do I find the information or what is the logic that tells me that app()->getStore() is now a method of the StoreManagerInterface class" ?
For example if my M1 code was
$users = mage::getModel('customer/customer')->getCollection()->getData();
What thought process or reference source do I need to lead me to the correct class to use in Magento 2 ?
Any pointers will be greatly appreciated