No guarantee about this, moreover it is related to online activities.
If the asset is stored in the exchange, it may have less safe, or risky. But I personally also use exchanges to save my assets because I am still trading them.
Key means the private keys or other phrases where we can use them to access our wallets. Commonly it is for online wallets, offline, and other types of wallets.
But for exchanges, we may not have the keyphrase, but we have some passwords/pins and other types of security modes.

If we want to store our assets for the long term, I will suggest you use a hardware wallet. But it will also depend on what assets do you have

And never try to tell others about the key, passwords, or even other secret information about accessing your exchanges or wallets. Learn by doing and you can make it