Reset

Short answer: If you followed the Apple reset procedure, you’re fine.

Long answer:

Lower-level details vary by storage and configuration.

Here is what Apple recommends doing to reset your Mac:

What to do before you sell, give away, or trade in your Mac - Apple Support

That erases the storage.

If you’ve used disk encryption has has been the default for a while, clobbering the key renders the user data storage inaccessible.

If you’ve used hard disk drive storage on the Mac and the Apple overwrite (and no encryption) very little or no data is left; only (potentially) around sector storage errors.

If you’ve used SSD storage here, just as soon as you delete the files or data such as with the reset and reinstall, the contents of the sector gets erased. This is required by SSD flash storage; each sector must be erased before re-use, and mechanisms such as TRIM and erase-on-zero are designed to keep the free storage pool large and avoid having to erase immediately before storage re-use; erasing before use is very slow. Erase on delete is vastly faster. (SSD storage also uses wear leveling, which means you cannot overwrite the same sector—the old HDD-era overwrites don’t work anywhere near the same.)

If you want to perform an additional erasure, you can either use Disk Utility from Recovery, or download and boot a Linux recovery disk or other such and use that to erase the storage. (Additional erasure on HDD is what most folks are familiar with, while an overwrite on SSD works wildly differently from HDD.)

If you want to verify, you’ll need another install and the ability to view storage contents directly; to scavenge storage. This works best if the storage was previously zeroed, as otherwise you’re going to need to know more about the contents and format of whatever was written to the Mac—the Apple reset procedure (above) erases and reloads macOS, so your verification checks would need to discount that storage when scanning. Or you’d need to zero that storage, and then verify.

If you’re doing something creative or your adversary is a nation state or similarly wealthy, this answer might vary, and you will want to discuss this with your site security officer or equivalent IT staff.

if your Mac has an SSD drive, in a few moments after data are erased, all trace are GONE.

What to do before you sell, give away, or trade in your Mac - Apple Support

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2015 models appear to have SSD boot drives.

The complete procedure for de-commissioning for re-use or recycling includes FAR more than just erasing the drive, as indicated in the linked article.