You're seeing just the function new, go back to Membrane.WebRTC.Track module for more information.
Link to this function

new(type, stream_id, opts \\ [])

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new(:audio | :video, stream_id :: String.t(),
  id: String.t(),
  name: String.t(),
  ssrc: Membrane.RTP.ssrc_t(),
  encoding: encoding()
) :: t()

Creates a new track.

Tracks belonging to the same stream should have the same stream_id, that can be generated with stream_id/0.