What makes everyone sound the way they sound and others sound different?

If you mean how come your voice sound different to you compared with hearing others, it’s because your ears hear your voice through the resonation of your skull.

If you mean why do people’s voices all sound different, it depends on a combination of voicebox size, tongue, mouth size, teeth, etc. etc.

I think it’s down to the shape/size of the vocal chords.

The size of the vocal cords determines a speaker’s pitch, and the shape of the tube determines the spectral shape of the sound. The shape of the tube, of course, depends on the exact position and shape of lips, tongue, nasal passages, teeth, throat, etc.

It’s possible (more or less) to separate these two effects with some digital signal processsing, so you can move a speaker’s pitch up or down to change male to female, etc., or remove the pitch information altogether, to change voiced speech to whispered speech.

If you leave the pitch constant, but change the spectral shape, you can make one voice sound like another of the same pitch.

The separation of pitch from spectral shape (i.e. vocal cord excitation from tube shape is a basic part of computer speech recognition and speaker identification.