Noting is secure nowadays. Neither any exchange or any online wallet can secure from theft. However, there is a silver lining. If you know how to configure a hard wallet, you most definitely should. it's the most secure way to store your crypto, though given that your hard disk where you store your hard wallet should not get corrupted. In order to prevent that, you should take multiple backups. Else one way is to evenly distribute your crypto balance across different online wallets. But refrain from exchanges. they are very insecure and most liable to attacks and thefts.

I use wallet exchange, I think it is very convenient and safe . But everyone chooses for himself what he likes

If you want to hold your coins for few years then use wallet. When you are a trader and you are trading everyday, then use exchange, because it is cheaper than sending a lot of transactions everyday.

I prefer save coins on a wallet to which I have a private key. I have nothing against exchanges, however, there were several situations when they failed and I was losing money.
Therefore, I believe that most of the coins are better kept on a wallet and part for trading on exchanges

Wallet that you own a private key is better and safer but if you constantly trade then put your funds in the exchange to save you from sending it back and forth. Put only those coins that you are holding long term in your wallet and it should offline wallet.

If you don`t plan to trade them, then is better not to keep your coins on any exchange. Indeed, if you have many different coins, it`s more convenient to keep all of them in a single place, but you have complete control over your funds only if you keep them in your wallets. Obviously, a hardware wallet would be the safest choice.

In my case I am storing btc in my wallet because it has feature where i can convert my fiat money into btc so whenever i want to buy btc or btc to fiat. its already in the wallet and also it has certain service where i can buy prepaid load and paying bills. So for now this wallet is useful to me. may I could use exchanger if i wanted to buy altcoins or vice versa.
 

 
 

Both are useful. Mostly, use cold storage for the larger part of your funds and then consider a wallet and some exchange accounts for the rest of the coins.

If you bought a coin for a long period, it is better to store it on your wallet, which has nothing to do with the exchange.