Shopify Partners: Get a list of items from managed stores

We are building a marketplace (currently not on shopify) with an express.js backend, and want to provide an easy way for merchants who are using shopify to easily upload and synchronize their inventories with our marketplace.

From what I understood, this is supposed to be solved with the Shopify Partner API, but I cannot quite figure out the steps. Here is what I've done up to now:

1. Create a Shopify Partners account
2. Create a test app (empty)
3. Add a Client's store as a managed store, with just the "View Products" scope
4. Mark my app as a Sales Channel on the Shopify Partners dashboard

At this point, I am confused as to what should happen next. What I want is a node (or really any language) snippet that gives me all items from all my managed stores. However, I have failed to find a comprehensive article on how to get there. To be clear, I do not want to sell products with shopify, I just want to use the shopify API to query our partner's inventory in real time.

EDIT: Added point 4. to the list and some clarifications.

You missed the whole section on creating a Sales Channel? That is what you are. A Sales Channel. So making yourself one of those Apps is what you do. You ask Shopify for your App to be a Sales Channel. Then you follow the guidelines they provide you. Your App has to conform to certain onboarding procedures, and then you get to work with the shops, and their products.