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Request Builder is a component of the Magento payment gateway responsible for building a request from several parts. It allows implementing complex, yet atomic and testable, building strategies. Each builder can have simple logic or contain builder composites.
\Magento\Payment\Gateway\Request\BuilderComposite is a container for a list of \Magento\Payment\Gateway\Request\BuilderInterface implementations. It gets a list of classes, or types, or virtual type names, and performs a lazy instantiation on an actual BuilderComposite::build() call. So that you can have as many objects, as required, but only those, which are needed for a request are instantiated.