I am trying to display in the UI (via javascipt) the existing PowerApps from my tenant. I couldn't find any references for some PowerApps APIs similar to PowerBi APIs. Can you give me a reference for the Graph API's you are talking about?
You can use the Admin connectors in either PowerApps or Flow. If using a Flow for instance, you can use one action that will retreive all environemtns, then use another action to iterate though those environments and list all PowerApps (and many detailes about them).