Check out this video for the actual process:
It's more daunting than you might think, if you have no idea how it's done yet, the modern 3D pipeline is pretty gnarly.
You have to first, separate the hard, non-organic parts of your model from the rest of the model. Increase the resolution of 'hard-surface' objects, using a package like Maya or Blender (recommend Blender since it's free, incredibly lightweight, and actually offers professional level tools, and more tools than some industry packages[Blender has Hardops and Box Cutter for instance]), increase the resolution of the hard parts using hard-surface modeling techniques.
For the soft parts, you would import them into Zbrush. You can begin sculpting on them immediately. But if you're taking this model from a game asset it's most likely going to have a lot of triangles in it, which is exactly what you don't want when working with Zbrush. You want a lot of square-shaped 4-sided polygons, that aren't thin or stretched. Instead of taking this object back into blender and remaking the entire model in quads, Z-remesher will automatically do this for you, but it will also eliminate the old topology of your mesh, and any other data bound to vertex points, so if you wanted to quickly texture your object, you would need to unwrap it again.
Protip: in Zbrush, typically, you want to add detail gradually as you are increasing resolution on your model. Start low, then add as much detail and general shape to your object at as low a res as you can. Then when you subdivide it, you add the most detail that you can and so-on, and so-forth. Don't try to divide your mesh 8 times, then start adding detail at that point. I know I linked you to a video above where the guy does that, but he's super experienced(and even then it's probably a bad idea).
For your hard surface objects, after you are done using hard-surface techniques to increase the resolution in an actual modeling package that isn't Zbrush, you can try importing the mesh into Zbrush to add more detail like scratches and what not. Ymmv, depending on how good you are at bringing a higher-res hard surface object into Zbrush in such a state that it can be sculpted on. HOWEVER, at this point, you may still be able to use z-remesher to even out the topology of the hard surface model, then sculpt detail onto it after that, but that might blur your hard edges, unless you take precautions against that using edge hardening techniques.
After ALL of this, you export all of the objects from Zbrush at a medium resolution, retopologize in Blender using snapping tools and the shrinkwrap modifier, unwrap the retopologized mesh, and use a tool like xNormal to bake the high-resolution model down to the retopologized mesh. And that's just getting the high-res detail, not the actual texturing process with normal maps, specular maps, etc.
As you can see, this is all way more complicated than it appears. If you are still going to try and have any more questions let me know