You've been watching too many old episodes of Hawaii Five O.
Data on
rotating rust hard drives is stored as a
MAGNETIC impression and for speed of access the data in each file is typically stored in easily identifiable sequential tracks and sectors. The mechanical read/write heads have an allowable tolerance (slop) so when the drive is overwritten a trace of the previous data may remain and can be recovered. For most DoD purposes overwriting the drive with alternating ones and zeros seven times is considered adequate protection. For deep black projects, used hard drives may be ground into fine powder as the ultimate safeguard.
SSDs store their data as on/off electrical
switches and have no moving parts so there is no slop in over writing data and therfore no possibility of recovering traces of previously stored data. Additionaly as all data locations on an SSD can, for all practical purposes, be accessed equally fast and to reduce
wear on the individual storage locations the data in a file is intentially scattered over the entire SSD rather than in nice neat consecutive locations. So only erasing the volume directory leaves you with literally billions if not trillions of unrelated characters, or in a 256GB SSD roughly 65,536.000,000,000,000 possible combinations. Given a multi-billion dollar quantum computer, and enough time it might be possible to recover such a drive. Do a full erase on that drive and all anyone would have to interpret would be an unbroken string of zeroes or ones and absolutely meaningless even with a quantum computer.
NOTE: add
File Vault and it won't make much difference whether you erase the SSD or not and then turn on
Advanced Data Protection and your data on iCloud is strongly protected as well.
