Also interested been looking into this myself. Some materials of potential interest:

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Official TikTok blog post describing with very limited detail how it works: How TikTok recommends videos #ForYou

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Papers:

• An Empirical Investigation of Personalization Factors on TikTok (2022) - sock puppet methodology to identify the parameters and their strength in influencing the algo
• Analysis on the “Douyin (Tiktok) Mania” Phenomenon Based on Recommendation Algorithms (2021)
• Trick and Please. A Mixed-Method Study On User Assumptions About the TikTok Algorithm (2021)
• Leveraging Rights of Data Subjects for Social Media Analysis: Studying TikTok via Data Donations (2023) - may be of interest

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WSJ did a video on them trying to reverse engineer the algorithm, not too technical though Investigation: How TikTok's Algorithm Figures Out Your Deepest Desires

Some blogs I came across that may or may not be reliable:

• Why TikTok made its user so obsessive? The AI Algorithm that got you hooked.
• The App That Knows You Better than You Know Yourself: An Analysis of the TikTok Algorithm