Hardware development comes in different phases. This article gives a nutshell of each phase.
PRD(Product Requirement Document)
PRD will allow the engineers to start doing the work on the hardware and mechanical level. You might end up with a mock up in order to see a non-functional design but the game is to reach something that some beta users can manipulate and that we could call MVP (Minimum Viable Product).
EVT (Engineering Validation and Testing)
Once your hardware is finalised, it needs to be tested and validated. This step is called EVT. The product cannot pass EVT if not all functional requirements are matched and if the hardware is providing the results and metrics that were expecting in the PRD through the specifications for example. Hardware and it will be tested for thermal, power, and EMI stresses.
DVT (Design Validation and testing)
The hardware will be produced in a small lot and will go through thorough test like dropping it from a certain height, burning it, check if it is waterproof, etc. Every country has different certifications and standards for products.
PVT (Production Validation and Testing)
This stage DFM (Design For Manufacturing) has been done and the moulds are ready, so is the stencil for your PCB (printed Circuit Board) by setting up a production line.This stage also consists DFA (Design For Assembly) as it can happen at different stages but most probably close to the end and it will allow engineers find the best and most efficient way to assemble the product after getting produced.