@Maciej Chmielewski Welcome to Microsoft Q&A Forum, Thank you for posting your query here!
StartCopyFromUri Underneath calls the Copy-Blob REST API. https://learn.microsoft.com/en-us/rest/api/storageservices/copy-blob
The limitations are explained here: https://learn.microsoft.com/en-us/rest/api/storageservices/copy-blob?tabs=azure-ad
Refer here for samples: H ow to use StartCopyFromUri can be found in our unit tests https://github.com/Azure/azure-sdk-for-net/blob/622c2e13a827cd9c6fc80a952a6292d84bf30dae/sdk/storage/Azure.Storage.Blobs/tests/BlobBaseClientTests.cs#L1817
csharp sample code to call StartCopyFromUriAsync() method to perform copy blob operation within same storage account
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