What is the difference between the collection and collection factory?
You can use "Collection" to load collection of perticuler module (e.g : customer) with all attributes,it will returns all attributes values.
While you can use CollectionFactory , you have to add create() method,generally Collection Factory is used if you want to filter data from given collection(e,g : you can filter customer collection with wholesaler group only ).
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I understand this follows the Factory pattern https://en.wikipedia.org/wiki/Factory_(object-oriented_programming)#PHP
So, create() method returns the requested object, in this case: \Magento\User\Model\ResourceModel\User\Collection
However... it seems you shouldn't be able to inject that object in constructor (Code 1), or at least, and responding to best practices, you should better inject the factory
Factories are service classes that instantiate non-injectable classes, that is, models that represent a database entity. They create a layer of abstraction between the ObjectManager and business code.
A collection is an instance of an object.
A collection factory is responsible for creating an instance of an object.
IE a collection factory creates a collection.
This is part of the Factory pattern: