What I mean is, not which app/option/button to use to remove a hissing sound, but how does the tool one uses to remove the hiss do that. Does it remove every low-volume noise? Does it break the sound into its components and remove a certain type of sound? etc.
Well, this depends strongly on the tool. In my opinion, the "easiest" (or at least the most intuitive) way to do it would be to use a filter that would remove the frequency content of the hissing sound. This could of course be done with the use of FFT-IFFT tools, but as already mentioned, this will strongly depend on the tool.
Additionally, it could make a difference if the "profile" of the hissing sound would be available to the tool or it would have to "guess" what a hissing sound is.
The best way to do it is to first examine a sample of the hiss. This means, you need to find a part where the hiss is the only sound that is being heard. If you have a denoising software, it will create the hiss profile, if not, you have to use a spectrum analyzer and manually check the frequencies that are increasing the most when the hiss is the only sound. Afterwards the denoising software applies a filter which is mostly an equalizer that cuts/reduces those frequencies. It can also be done Manually if the hiss is not too complex or doesn’t get in the way/conflict with many other sounds in the same frequency range. Of course this will also affect the sound as a whole. That’s why a clear source file is always the best.