Dear Z.M.123,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
As per your description, it seems that you are having concern with Power Automate, APIs and SharePoint integration with Loop.
For the concern about Power Automate, considering that we mainly focus on general and build-in queries in Loop, we're afraid we may have limited resource for Power Automate flow, since we have a dedicated channel Microsoft Power Automate Community, for you to be assisted properly, we sincerely recommend you go to that community and post a new thread. Members and engineers over there are proficient in the knowledge of Power Automate queries. They will focus on the specific situation on your side and provide specific suggestions for you.
For the concern to upload data and files from separate SharePoint, generally, upload file in Loop uploads file from your system file picker currently.
For your reference: Upload files to Loop workspaces - Microsoft Support
For the third concern about API for Microsoft Loop, we've done searches, however we have not found API for Microsoft Loop announced in the Official article currently, if you're willing, we'd recommend you post a new thread in Microsoft Graph - Microsoft Q&A for more dedicated help.
Apologies for redirecting you to a different community, as members of the published category focus on built-in queries with Loop and have limited knowledge about APIs and Power Automate; So, to get quick and better assistance, we redirect you to the right one.
If the understanding above is not consistent with yours, please feel free to post back and point that.
Sorry for the inconvenience again. Your patience and understanding are highly appreciated. Hope you stay safe and healthy.
Sincerely
Rhoda | Microsoft Community Moderator