Secure free space and disk encryption

Hello everyone

what is the recommended command/app to secure the free space and make them unrecoverable ?

if the disk is encrypted. does i still need to secure the free space and to secure how i delete file or not?

let say disk was encrypted then i reformat the whole disk can the data be recovered? and if does is it will be like encrypted files so if i have the key it can recover the file to it unencrypted form ?

where the keys stored ? if i removed that key would it be unrecoverable ?

sorry for my confusing about those stuff and thanks for your time and help :slight_smile: