Some parts require more complex methods: two or more part mold, Flexible molds, or investment mold. The most common cause of these needs is cast part would not release from the mold without damage to the mold or part.
Undercut, high detail or complex shapes need two part molds. The space between the part highlighted in blue and the space below is undercut.
This is an example of a 2 part mold of a "round" object. The cast is in brown wax.
Another way to handle undercut is a rubber mold that can flex over the undercut or a split mold. The split mold is made by cutting the mold to give better demolding. But because rubber is flexible it will need support during casting or deform causing quality issues.
The investment mold process is one time use mold. The mold and the casting will be not survive.
The mold master is often made of wax, foam or PLA. The mold is often made of plaster, green sand, or special casting compounds.