Crypto assets are stored in offline, cold, multi-signature wallets.
Transaction signing only happens offline on separate devices that have never been connected to the network, and this process involves several people. The multi-signature process involves several keys (N) with a required quorum of any (M) keys.
For example, you need 3 out of 5 keys or 5 out of 8 keys to conduct a transaction. Thus, it's not possible to sign the transaction with a single key. If one of the multi-sig keys gets missing, you won't lose control over your assets completely.
We store encrypted parts of the keys in the banks' safe deposit boxes to prevent potential loss of the keys due to natural disasters, including floods, earthquakes, fires, etc.