There are two main concepts related to SSIS internals which need to be understood before we deep dive in optimizing SSIS packages.
Transformation - There are different kinds of tranformation in SSIS which overall impacts the performance of SSIS.
Execution Tree - At run time, the data flow engine breaks down Data Flow task operations into execution trees. Execution trees are enormously valuable in understanding buffer usage.
I have written an article covering different kinds of transformation and how data flow task operations are divided into execution trees.