The Weather Station code builds on Nordic SDK Eddystone Example.
In addition, Ruuvi’s drivers are used for communicating with the onboard sensors.
Step-by-step video describing every aspect of the development would probably be days - if not weeks - long.
Please open a thread tagged as firmware development with a more specific questions such as “how do I set up development environment” or “how do I deploy my own code to RuuviTag”.
If you’re looking for more comprehensive information, Nordic Developer Zone has a lot of tutorials on how to program for nRF52 which is the core of the RuuviTag.
Nordic SDK also has a lot of example programs, most of which can be made to run on RuuviTag as-is. We will post a detailed tutorial on how to port SDK examples to RuuviTag in a few weeks.