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Steam Microtransactions (In-Game Purchases) are handled via the Steam Inventory feature. We have multiple articles and a significant number of tools set up around Steam Inventory.
The Steam Inventory system can be used for a number of things not just MTX. If you are interested in using Steam for MTX then it is Steam Inventory that you will be using to define your items, establish there prices, start purchases and detect what items the user owns.
Have a look at our articles on Monetization before you get started designing your games monetization systems.