What does .upload do?

I was recommended to use Uppy and I’ve been checking its docs.
I saw there is this .upload() method, but I have no clue where it uploads to.
I saw when configuring the Uppy instance, you have the option to specify the backend url, but at no point does it use queries from the client server.

In short I have a mutation sendMessage where through graphql it accepts the conversation id where the message belongs, the text of the message and the uploads. It is a mutation so it has a mutate(variables: {...}) function through which the changes to the API are made. So in the react component, when the send button is pressed, it calls a local function that calls this mutate function.
With Uppy I have no idea how any of this would change. Any advice would be appreciated, since it really looks like a fantastic library, but with no coding examples on how to interact with my personal API/Database, I am at a loss.