no special software needed, just make a graph that tells a story.

It's a question of personal preferences. I try to include the more visual highlights of the paper - an image, a graph, a structure, etc.

I tend to use Powerpoint but that because my graphical software abilities is very limited (I used to use Freehand SW many years ago, bun then Adobe bought and killed them)

I agree with Yoram Gerchman powerpoint is the easiest way and can do almost anything.

Even though people tend to ease the burden of preparing a graphical abstract by using PowerPoint, Word, or even Paint, this approach doesn't suit me. Academic publications are an extremely serious job, and every part of it must convey that seriousness. Paint, PP, etc don't seem professional, most of the time. But some can create miracles with PowerPoint that is for sure. Some may even hire a professional graphic designer.

For me, it should be done using simple yet robust software, and I prefer to allocate extra time to prepare visuals that are both rich and simple enough to convey the essence of the work done.

My weapon of choice is the open-source software: InkScape. I sometimes combine a 3D drawing with 2D. Inkscape is 2D and for 3D I use Tinkercad. All is free software and easy to get familiar with.