YouTube API - how to fetch all playlists of a channel, even the ones who were added from other channels

I stumbled across this problem as I wanted to transfer all playlists (my own and those who I added from other channels) to another channel.

The code I use returns only my own playlists and not the ones I added to my channel from another channel.

I tested this behaviour in the target account which contains a private playlist I created and two playlists from another channel. The script I created uses oauth in order to 'see' my private playlist, but is not able to find the certainly public playlist from the other channel.

My code:

    request = youtube.playlists().list(
        part="snippet,contentDetails,status",
        channelId="UC_x5XG1OV2P6uZZ5FSM9Ttw",
        maxResults=25
    )

maxResults does not play a role here as I only have got all together three playlist on the test account. The playlist from another channel is clearly visible when I use the "Library" link and scroll down to "Playlists".

I would appreciate any hint to solve this 'mystery'. TIA!

---

This is how I added the playlist from the other channel:

• go to the other channel
• list the playlists
• click on "VIEW THE FULL PLAYLIST"
• click on the icon which looks like a sandwich combined with a plus sign (hovering on it gives 'save playlist')

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