- Someone come up with what something should do when X
- Designers create the UX and UI to support that feature
- Developers start reviewing the UX/UI, create a list of things designers/product managers forgot to think of when doing the design/product requirements
- Designers finishes the design
- Developers manually try to copy the design into code. How this happen depends a lot
In 90% of the companies, this is how I've seen it being done. Some places are trying to make that last step automatic nowadays, but the progress and results are nowhere near production-ready yet.
Let me know if you have more specific questions :)