what does closed source mean? do they not share private keys with third parties? if that's what it means, but why was my personal wallet trust wallet hacked by someone even though I never gave or told anyone the private key and they stole all the assets in my wallet, even though I had installed all the security.. is there a great hacker who can penetrate the security of trust wallet or is the security system less secure?
Closed source mean the code can't be viewed by anyone because the developers hide it.
If the wallet is open source, anyone can see the code and someone can verify whether it contain malicious code or something like that. So, if you're using closed source wallet, you must trust 100% with the developers, however this ruin Bitcoin idiom which is "don't trust, verify". No one will know if the developers share the private keys or not with third parties since it's closed source.
This one is the example of closed source
https://github.com/trustwallet/wallet-core/blob/master/android/.gitignore