VH
height: 100vh; means the height of this element is equal to 100% of the viewport height.
example:
height: 50vh;
If your screen height is 1000px, your element height will be equal to 500px (50% of 1000px).
CALC
height: calc(100% - 100px); will calculate the size of the element by using the value of the element.
example:
height: calc(100% - 100px);
If your screen height is 1000px, your element height will be equal to 900px (100% of 1000px and minus 100px).
*I think your former developer didn’t need to use calc() if he/she didn’t want to calculate value.