Hi @Jacob Brock-Hansen ,
Do you want to use API to store and retrieve files in SharePoint Site? If yes, you can use the REST API to achieve this. Here is a document about using REST API to store and retrieve files, you can refer to it. Hope it can help you.
Document for reference:
https://learn.microsoft.com/en-us/sharepoint/dev/sp-add-ins/working-with-folders-and-files-with-rest#working-with-files-by-using-rest
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