I play the Indian classical violin. In that, when you have a song in A, for instance, you would tune the first (lowest) and third strings to A, an octave apart, and the second and fourth to the perfect fifth, again an octave apart (that's E, if I'm not mistaken). Similarly, if your song is in F, you tune the first and third to F, and the second and fourth to the perfect fifth, C. After that, you use the Hindi/Urdu equivalents of Do, Re, Mi, Fa, So, La, Ti, and Do.
However, since I believe that this platform has primarily western musicians, I'll try to translate stuff to your lingo. Tell me if something doesn't make sense; I'll review the little I know of western notation.
Let's consider a song in C major. The notes should be C, D, E, F, G, A, B, C; with my tuning, I will play the first C on the third string with no fingers on the neck of the violin, and G on the fourth string with no fingers, and I'll press the fourth sting somewhere around the middle of the neck for the final C.
No, I switch to some other scale within the same string tuning, so I play the notes C, C#, E, F, G, G#, B, C (I don't know the western name for this, but it certainly exists and is very common in Indian music). To play C# and G# on the C and G strings respectively, I place my pointer finger right next to the end of the neck, so the length of the vibrating string is changed very slightly. This creates an increase by a semitone. However, if I were to play a different common Indian scale, with the violin tuned to CGCG again, such that I had C, D, E, F#, G, A, B, C, I would shift my ring finger (which I use to press both F and F#) up but almost a whole inch compared to the F position to get F#. And if you change the tuning of the violin, as long as you keep the alternation of root-perfect fifth-root-perfect fifth, the finger positioning doesn't change at all for corresponding Do, Re, etc.
On the other hand, with a guitar (which I don't know how to play or tune), I think each fret represents a semitone, and for the most part, they seem equidistant. What's going on here? This somehow seems confusing and it doesn't add up. Is it because of some physics, or is it because of the way notes' frequencies differ somehow?