Is this NSW?
I’ll assume it is.
Unless there is specific wording on the strata plan or an exclusive use by law this is how the lot owners property is defined.
It’s the cubic space between the inner walls and inner boundaries as delineated on the strata plan
Everything else is common property
For a 5 year old building A thick line drawn on a boundary is taken to be on the inside of the boundary.
Same with floors and ceilings. It’s the space between them that’s owners property.
( just to be clear, when you buy into strata , you buy air, which is your own, and then a share of the common property)
In your case it’s the responsibility of the OCto repair the cracks and leaks. The costs of the repair is borne by all lot owners in proportion to their unit entitlements.
However you may want to get a move on and envoke your building defects warranty against the builder/ developer. It’s within the defects warranty period of 6 years. Get you strata manager to start a claim ASAP.