1. How to stay anonymous when you buy and sell Bitcoin?
It's pretty difficult to be totally anonymous, but you can maximize it by purchasing through non-KYC exchanges such as HodlHodl[1], LocalCryptos[2], and Bisq[3]. That way, only you and the person you're transacting with(and the platform) knows about the transaction.
2. How to anonymize Bitcoin if you purchased it from a centralized exchange?
Run them through a mixer(Wasabi[4], Samourai[5], JoinMarket[6]) or a Mixing service(ChipMixer[7]). Along with that, basically privacy guidelines such as not reusing addresses and coin control.
Besides this, I would like to find out what is the best anonymous bitcoin wallet for 2021. Please share your top privacy-protecting bitcoin wallets and highlight the reasons they this or that wallet in your top.
Pretty much run Bitcoin Core[6]. Not the most user friendly, but if you really want privacy then go use it. You can run it through Tor which is a huge plus in terms of anonymity.
[1]
https://hodlhodl.com/[2]
https://localcryptos.com/[3]
https://bisq.network/[4]
https://wasabiwallet.io/[5]
https://samouraiwallet.com/[6]
https://bitcoin.org/en/bitcoin-core/