I’m trying to download electrum to store my Bitcoin as I no longer want to use blockchain anymore since it’s not well secured. I don’t know if I should check on App Store and download electrum from there or I search on goggle?
Since you referred to App Store, I guess you are looking for IOS version of electrum.
If that's the case, note that electrum isn't available for IOS.
You can visit the official website and download electrum for Windows, macOS and Android.