Can someone explain what the underscores mean, it is something about ommitting the argument label, but the arguments are labeled here arent they?

You don't need semicolons in Swift. :)

All the solutions are like that; the example test cases are 'just to get you started'. They aren't actual test cases.

Can anyone explain why while this solution pass the test suit, times out the example?

This will cause a new allocation and string copy for every character, which is quite inefficient. You know how big the resulting string will be at the start, so you should just allocate one buffer for it and copy the input string into it once with the desired replacements.

It will also leak every buffer that's allocated except the last, which is returned to the caller. If you want to resize a buffer, you need to malloc a new one of the desired size, copy the contents from the old one and then free the old one. Or just use realloc.