Rest API Fetching details

I have a major requirement from my manager and this needs to be fulfilled as early as possible.

Requirement: There is a List A and List B. List A acts as a database from which data has to be fetched into List B.

Now List A has three Columns :

  1. Business Unit(BU)(Dropdown)
  2. Project Name(Textfield)
  3. Should Cost Modeler(People Picker Field)

Now, i want to get all my projects based on Business Unit and then when i select a particular project it should fetch employee name in regard to the project.

Example : If in List B i select a Bu : Xyz , then all the projects pertaining to the BU should pop up and then as soon as i select project name, the should cost modeler name should be in dropdown.

How do i fulfill this requirement. As it is a very tricky one and i am not so good in Rest API.