Looks like it doesn't upload the file, just calculates its hashes client-side, and passes them to some subtitle search API.

    POST https://suub.onrender.com/api/subtitles/search
    {
      "hash": "cf5d493763954141",
      "subdbHash": "e930e3833542f4fce3dd041c957e8de8",
      "napiHash": "6ccaa1b4e5b6778812ac841b7a35ab18",
      "fileSize": 63816128,
      "fileName": "recording.mkv",
      "language": "en"
    }