Okx and Binance are centralized crypto exchange they are not Bitcoin wallet they are platform where you send or buy bitcoin and it’s access it’s not entirely your own, you can be seeing audio and automated amount.

You can't buy bitcoin directly from Electrum; they don't have that option, but Electrum is a good wallet to use and store bitcoin for a very long time, provided that you can safely guide your private key. 
 
All you need to do is go to any exchange, either a DEX crypto exchange or a centralised exchange. Buy Bitcoin and go back to your Electrum. generate your wallet, make sure you back up the phrase to the wallet and be sure it's correct, and then you can withdraw directly from the exchange to the electrum wallet, where you can be sure it's safe and you have full access to the self-custodial wallet.