Electrum was used on which OS ?

Do you have your electrum  in your main computer ?

You should keep those huge amount in a separated wallet, cheap computer, used only for this purpose and disconnected from internet  (Linux as OS to increase the security)

How they did ? Can't say, but most probably by a backdoor in your computer... You can't trust the security of any OS today, 0day are used by many shady security group, gov and more...

Simple fact today, you want your device / computer secure ? Don't connect it to internet (In your case, don't let a huge amount in a wallet always accessible by internet)