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MMFS said:
rotoscope ??
doesn't look like it when watching the making of
Might not be rotoscoped but definitely looks heavily referenced
According to her YouTube account, she is known for her watercolor-like rotoscope work, so I'm thinking this is rotoscoped as well.
https://www.youtube.com/user/shishinkooo/about

Edit: This confirms it. She mentions it herself in the interview.
https://www.youtube.com/watch?v=LwJvcr3rVj4&app=desktop