A few brands tailor their pants in such a way that it is easy for people with prosthetic legs to put them on. This is how they ___________.

'Go the extra mile' means to make an effort beyond what is expected, which is what the brands do by tailoring pants for people with prosthetic legs. The idiom is fixed: 'go the entire mile', 'go to the extra level' and 'go to the extreme level' are not English idioms.