Mirroring objects is pretty easy, a simple tutorial below :)
(i showed this for a sketch, but it works the exact same way for mirrored parts aswell, in that case you usually just use planes)
create the sketch
select the sketch entity youd like to mirror
select the mirror line, you can see the preview of the mirror as a yellow circle
since the mirror distance depends on the sketch/plane, you can add a new sketch with a defined distance from the original mirror line. when you increase/decrease the distance between the initial mirror line, the mirror moves along with it.