You're seeing just the callback fetch_access_token, go back to WeChat.Storage.Hub module for more information.
Link to this callback

fetch_access_token(appid, args)

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fetch_access_token(appid :: String.t(), args :: term()) ::
     access_token: term(),
     expires_in: term(),
     refresh_token: term(),
     timestamp: term()
  | {:error,
       __exception__: term(),
       errcode: term(),
       http_status: term(),
       message: term(),
       reason: term()

Fetch access_token of WeChat common application.