Both of these questions have the same answer. To undertstand, you have to consider a “lot” as anything within the (usually) six-sided box formed by the wall ceiling and floor of the lot. The walls ceiling and floor are common property – the airspace inside, and anything contained in it, is lot property. And anything within common property is the responsibility of the owners corp.
So, on the face of it and unless special circumstances prevail, the pipe in the floor of the garage, and its roof an walls, if they are common property, are the responsibility of the Owners Corporation.
We can’t tell you for certain that they are because your strata plan and by-laws may be different – but they probably are.