A phone or laptop that's being sold, donated, or simply handed down to a family member is easy to think of as "done with." Whatever's still on it, though, goes wherever the device goes next.
What's actually still there
Saved passwords, old text messages, photos, email accounts still logged in, sometimes years of it, sitting quietly even after the files you noticed are deleted. A factory reset genuinely clears this, but only if it's done properly, not just by deleting a folder or two.
Deleting a file and wiping a device are two different things, and only one of them is actually safe.
The proper way to do it
Back up anything you want to keep first. Sign out of accounts, particularly your Apple ID or Google account, before you reset. Then use the device's own factory reset option, not just a manual delete, and confirm it's actually finished before handing it over.
Worth building into a habit
This matters just as much for a business handing down an old work laptop as it does for a personal phone. A quick, consistent process, the same steps every time, is worth more than remembering to "be careful" in the moment.
Straight from the source
Apple, Google, and Microsoft all publish official steps for wiping a device properly. Find them on the Free Resources page.