Explain how to find the cubic area of a room and the whole building.

Well, first lets get our terms straight.
There's no such thing as "cubic area".

-- The distance along a road is "length".  That's how much string you need
to stretch from one end of the road to the other.

-- The amount of surface on a sheet of paper, a floor, or a front yard is "area".
That's how much paint or carpet you need to cover it.

-- And the space inside a balloon, a milk jug, or a room is the "volume".
That's how much water it takes to fill it up.

To find the volume of a room, measure its length, width and height,
and then multiply them all together.

           Volume = (length) · (width) · (height)  of the room.

If you measure the dimensions of the room in feet, then
the volume will be in cubic feet.

If you measure the dimensions of the room in inches, then
the volume will be in cubic inches.

If you measure the dimensions of the room in meters, then
the volume will be in cubic meters.

You could do the whole building the same way ... multiply its
(length) x (width) x (height) ... but there could be a big problem
trying to do that.  You can only do it that way when the space is
the shape of a rectangular prism.  That's the shape of an ice cube,
a Rubik's cube, a brick, a cigar box, a pizza box, a suitcase, a
shoe box ... things like that.  If the building isn't the shape of a giant
brick or a shoebox, then it probably looks like a stack of several blocks
with different sizes.  It might even have some curves on it, so you
can't just pick a length, a width, and a height and multiply them.
In that case, if you need to find the volume of the whole building,
you have two choices:

Choice #1).  Get a drawing of the building, and see if it's possible to
break it up into smaller sections, with each section shaped like a brick
or a shoebox.  For each section, multiply the (length x width x height)
of that section, and when you've done that for all the sections, add them
all up.

Choice #2).  Measure the length, width and height of every room
in the building, use those to calculate the volume of each room,
and when you have them all, add them all up to get the volume of
the whole building.