Although most of us know how to securely erase a HDD, the following article also shows how to do it on a SSD. How to Use Mac's Disk Utility to Securely Wipe a Drive

I would add to OWC's comments on erasing an SSD, If the drive is formatted APFS each file's data is intentionally scattered all over the drive which makes any attempt to re-assemble the contents of any given file impractical if not impossible.

Isn't that the case with an SSD formatted HFS+, too?

Yes, but as I understand it, not to the same extent as in APFS. For one thing there is an additional virtualization layer in APFS. For example you can have multiple APFS volumes on a drive and theoretically each would have the maximum capacity of the drive available to it. Roughly analogous to having multiple sparse disk image or sparse image bundles on an HFS+ volume.

As its pieces fall into place, APFS gets more and more interesting...if not fascinating.

Thanks.