It certainly is possible, but it’s much easier when you have the originals and not just the STLs. Not sure how much you know about 3D modeling, but most programs save their files with a bunch of metadata attached in their own propriety filetypes such as .SLDPRT for SolidWorks, .f3d for Fusion 360, .SKP for SketchUp. These files are generally a lot easier to modify because they contain history of the timelines and/or the 2D sketches that you then extruded to be 3D, and they have certain things defined that .STL files cant – curved lines for example.
STL was created for the 3D printing world as far as i know, as evidenced by its namesake stereolithography. It’s a representation of whatever you modeled in your design software, but it’s not the same thing. It uses triangles and only triangles to represent the outer mesh of your designs, and as such can never get to the level of detail that your models can (i.e. curved lines). You can see evidence of this in the nice compound curves of the part you linked
It’s definitely good enough for 3D printing, but the result is that it can make it hard to edit because the nice surfaces and angles and circles that used to exist don’t anymore and it’s a mishmash of random triangles.
A good analogy is that you have a detailed vector file which you then export to jpg or something. From afar, they look the same, but it’s much more difficult to edit the jpg than the vector file. Sometimes the software is smart enough to backtrack and regain some of the detail thats lost (i.e. the image trace functionality in Illustrator) but not always.
Anyways that’s all to say that it can be difficult but not impossible, and I’d recommend Fusion 360 as a good free 3D modeling software to try it out if you’d like.