Unit tests are created during the development of software, they test various components of code to ensure it works as intended and there is fault tolerance in case for example expected inputs are not given. Users do not see unit tests, in your analogy a better example would be a mechanic using his own tools to check if the oil is low and the red indicator light will work, however the red light itself is more akin to the web site the user sees, the tool the mechanic uses would be the unit test they developed to check the mechanism of the red light would work correctly. Unit tests are a best practice for software engineering that an experienced senior technical lead would put in scope for a project. However, it’s one of the many things in software development non technical folks may not understand well enough to appreciate why more time is needed to deliver a product.
