that after a certain amount of time the bitcoin becomes practically anonymous? As in, not being able to be traced back to the original KYC exchange, or in the case of "tainted" bitcoin, that?
As others have already stated, the simple answer is no. If you care for coin anonymity, you should check out mixing services. Ideally open-source wallets with so called whirlpool features. See the wallet section of this forum. This is a good start: https://bitcointalk.org/index.php?topic=5457078.0
Generally I would recommend to check out Andreas Antonopoulos as a first resource to get informed about some Bitcoin fundamentals. https://www.youtube.com/@aantonop/videos