I have an SSD that has 3 partitions and it was probably a windows install but the largest partition shows no signs of what was it or how could it be mounted (basically It's universally recognised as raw/unallocated).
While I had some success with Testdisk on previous drives, I couldn't get it to find anything on said drive, and went to look for other alternatives, and came across DDRescue.

Of course I could use the original, but a gui version is still tempting as it's probably easier to use, but all I could find is one guy, and his website where I could buy the software (https://www.hamishmb.com/ddrescue-gui/). On the site it mentions that it is open-source, but I found no signs of any source code anywhere, let alone packages.

I have also seen threads about having to add the guy's PPA to apt but neither does that have the package.

As for my system (if it's even relevant), it is Debian 12.

What am I getting wrong? Is there something I may have missed?
Thanks for the replies in advance!

ddrescue is a utility to make an image of your drive. There are a lot of utilities like this (dd dcfldd, etc). ddrescue is a very good one.
If you like a gui program on linux to create a disk image, you might try guymager. It's in the debian repo.

testdisk is a utility to operate on a disk, or better, on a disk image created with one of the forementioned tools.
It can find deleted partitioning and more. photorec, which is part of the testdisk suite can recover deleted file, even from a raw disk.

So ddrescue and testdisk serve completely different goals. It's best practice to first make an image and then do the analysis on the image.

Be aware that photorec cannot recover files that were on an encrypted partition. In that case you need to mount the partition first and then use photorec on the partition. What you see as a raw partition might be an encrypted partition.

If you have a Windows machine, install FTK Imager (free utility) on it and take a look at the drive with that one (i.e. load a physical drive). FTK Imager does a nice job on FAT and NTFS partitions. It can also create an image (like ddrescue), though ddrescue does better quality on damaged disks.

If you have created a diskimage, you can analyse that with lots of tools: Testdisk, Autopsy Sleuthkit, FTK Imager...

Hope you can do something with this...

My thoughts also.

What does "lsblk -f" show ?.