As many users have recommended the use of https://kycnot.me/ for the selection of decentralized exchanges that do not require KYC, I am sure they have provided you with enough information you need regarding the selection of a decentralized exchange to use for your Bitcoin purchase.

All you have to do is choose the one you want, download their application on your PC or phone, and proceed.

If you want external security, you can send your Bitcoin to any Bitcoin wallet of your choice from here after buying it from any of these recommended exchanges if you want to hold the Bitcoin for a very long time.

If you want to trade, Centralized exchanges like Houbi, Kucoin, MEXC, and others have good trading features, but make sure you send your funds out from the centralized exchange after trading.