Fiberglass parts are often made with the use of a mold. This mold can be reused to create additional fiberglass parts.
Very often these molds are created with fiberglass. The parts are also created with fiberglass. Release agents are used to ensure that the part will separate after it is cured.
Sometimes single molds are created from several separate parts.
This video below demonstrates the use of a fiberglass mold (that was 3d-printed) and the making of a hand laid fiberglass part from that mold.
It is very informative and well done!
from Youtube Michael Curry