I think many companies engaged in blockchain provide online wallet services for free, such as blockchain.com which I think has a fairly large level of security. You can add some two-step verification to your wallet.
if compared to online wallets and offline wallets, the security level is better online wallet
I don't agree with this statement.
Even if you pick a strong, unique password and add 2FA authentication, this will NOT help against any security flaws introduced by the operators of the online wallet/exchange.
If the exchange has a bug, you can lose everything
If one of the exchange's employees is dirty, you can lose everything
If the exchange goes bancrupt, you can lose everything
If the exchange turns scam, you can lose everything
No type of 2FA authentication can save you from these issues... And these issues are not neglecitible nor are they marginal... Above issues happened dozens of times before, and they'll happen dozens of times in the future.
Next to these attack vectors, you're also more vulnerable to phising (if you're not using an online wallet, you're not vulnerable to a mail urging you to sign in to a phising site) and if your pc gets compromised, the level of vulnerability of an online and a desktop wallet are about the same.
I just don't get why people insist on using thirth parties to store their BTC, and in doing so are fine with all the extra attack vectors they expose themselfs to...