"the old Mac is dead because the hard drive failed. I want to access the hard drive's files. Surely there's an adapter that exists."
If the Mac is "dead" because the internal SSD "failed" -- then it's not likely you're going to get anything off of it. Many times when an SSD "fails", it does so catastrophically, "never to be seen again".
If it was something that went wrong with the software on the drive -- that made the Mac unbootable -- then it -might- be possible to get something from the drive. See below.
There aren't many enclosures for these.
The connector is proprietary to Apple.
The only one I know of may be from Other World Computing, and it will run $75.
It might work, but again, if the SSD's circuitry itself has failed, you may "get nothing".
Like Clint Eastwood said... "Do you feel lucky??"
Think hard... is there anything on the old drive you really, REALLY need?
One last thought:
If the SSD is still IN the 2015 MBP, try this:
Boot to INTERNET recovery:
Command-OPTION-R
at boot.
You'll need your wifi password, the internet utilities take a while to load, so be patient.
Can you get to the internet utilities?