If you want to sync inventory using APIs from Shopify and another platform, you'll need to use an app. The app will needs to be designed specifically for consuming data for the APIs you're using. You can check the Shopify App Store to see if one already exists, or check with your existing platform to see if they can offer something.
If you want to build an app yourself, I recommending having a look at our tutorials to help you get started. If you're looking to hire a developer to build an app, the best place to start is the Shopify Experts marketplace.