Resources/methods to construct STL files using disparate parts using OpenSCAD
I'll offer up the following options, without waiting for an answer to my question.

One can import a specific STL file into OpenSCAD and perform rotation, translation and scale, but it would be "clunky" as you'd be making miniscule changes to the parameters for those actions. Each imported object would have to be manipulated individually via the changes in the code. If your imported objects have quantifiable dimensions, that is, if you know specific measurements, you would have an easier time of the project.

If you are not limiting yourself to OpenSCAD, I'd recommend the easy-to-use MeshMixer. I recently "assembled" the parts of a cosplay "weapon" for a friend. All the parts were individual STL files and were positioned in 3-space using MeshMixer to make the parts appear as if the project were printed and assembled.

The transform tool in Meshmixer is a graphic three-arrow/three-axis manipulator icon. Grab an arrow tip and the model moves in the direction of the drag. Grab an arc between two arrows and the model rotates. If you want five degree rotation increments, move the mouse outward after the grab and get a calibration circle. When the mouse is on the circle, you get snaps to five degrees. If you need smaller movement, move the mouse out even farther from the center.

The 3-space presented in Meshmixer gives you the ability to align pieces/parts in all three dimensions, to your satisfaction. There is a model hide and a model-ghost feature as well, to assist the placement.

I expect that one could use a program such as Blender, but the learning curve is more like a chasm or cliff-face.

One could use an engineering-focus program such as SolidWorks, Fusion 360 and so many others, but I think your head, arms, legs, torso reference means you aren't using orthogonal models more suited to those programs.

Meshmixer, like OpenSCAD is free. I use both and approve this message.