Not really any special program. Photoshop, Affinity Photo, probably even Gimp, or any editor that has a smudge tool.
Adjust your smudge settings, use different sized brushes and stroke techniques, use small brushes, multiple layers.
It takes time, not something that is just easily rushed and regurgitated in an hour or less for some quick fg.
Start off with a simple base smudge to get the shape of what you want, you can always add color or adjust it later.
I generally start with a grey background and toss down black blobs and lines on a new layer, then smudge them out.
After that, you either keep adding more layers of smudge, or add in color, play with filters and gradients, and lighting.
Other times, I used the actual render, made a copy, and smudged the render itself to create the background.
Here's something I was playing around with and experimenting different things, honestly not my favorite outcome, but it's all a learning process.
Imo the 2 major issues I have with this is the lighting, and the fact that it feels like a render sitting on top of a background rather than being worked into it.
Here's another one I did a couple months ago
This isn't a smudge, but almost looks like one, it's mostly just playing with filters and layers, but I really like how it turned out
Used this as the base image
https://i.imgur.com/oI9xben.png