This is the step most people skip, and it's the most important one. A factory reset on a phone or a format on a hard drive does not destroy your data. It removes the file system index — the table of contents — but the data is still there and recoverable with free tools. Before any device leaves your possession, do this properly.
For iPhones and iPads: Settings → General → Transfer or Reset → Erase All Content and Settings. On a device with a Secure Enclave (iPhone 5S and later, all modern iPads), this is cryptographically secure — the encryption keys are destroyed and the data becomes unrecoverable. This is the one case where factory reset is actually sufficient.
For Android: enable encryption first (Settings → Security → Encrypt Device), then factory reset. On modern Android with file-based encryption (Android 7 and later), the encrypted data is unrecoverable after reset. On older devices, do the encrypt-then-reset sequence to be sure.
For Windows laptops with an SSD: Settings → System → Recovery → Reset this PC → Remove everything → Change settings → Clean the drive. This does a secure erase on SSDs that support it, and overwrites data on those that don't. For HDDs, use DBAN (Darik's Boot and Nuke) for a full multi-pass wipe.
For Macs: Apple Silicon — Shut Down, hold the power button to enter Recovery, choose Erase Mac. This is secure. For Intel Macs, boot into Recovery (Cmd+R at startup), use Disk Utility to erase the drive with the Security Options set to at least one-pass overwrite.
If you're disposing of a hard drive and you don't need it to be functional afterward: drill three holes through the platters. Unrecoverable. No software required.