In general with injection molded parts the process has a few steps:
1. Design
Starting from a set of requirements and what has to go into the enclosure you start working on the design. Here you are often working with other teams that influence the design. These could be other mechanical designers working on interior components, electrical designers working on PCBAs, industrial designers that want to make the exterior aesthetically pleasing and nice to interact with.
The parts is designed with many factors in mind. What material will you be using? How does the part take into account good injection molding design practices (consistent wall thickness, avoiding undercuts, drafts to enable release from the mold, etc.)? How do the parts mechanically fit together? Normally you will be making prototypes at this point (e.g. 3D prints).
Once you are happy with the design you create drawings and send a 3D database and the drawing to vendors for quote.
2. Design For Manufacture (DFM)
Once you get the quotes back and choose a vendor you start the DFM process with your supplier. Their tooling engineers take a look at your design and provide a document with their feedback telling you where your design needs changes in order to be manufacturable. Part of this is running the part through simulations where they simulate how the plastic fills the mold to identify potential issues. You go back and forth until both parties are satisfied with the design before giving approval to start tooling.
3. Samples/Testing
About 6-8 weeks later the vendor has finished the tool and done some tooling trials to dial in their process before sending you the first off tool (T0/FOT) samples. At this point the parts are generally not textured and may have some defects. You inspect the parts and provide feedback about what needs to be improved. At the same time the parts will be tested to see if there are any design issues, if something needs to be changed there you issue the vendor a tool mod report and they quote the costs to make the changes in the tool before sending updated samples for inspection/testing (usually called T1, T2, etc.). Once you are satisfied you have the vendor send the tool for texturing and get samples that are hopefully the final design and appearance.
4. Production
Once the design is done approval is given to start buying the parts in high volumes for production.