Welcome! I’ve always wanted to make something Pokemon-like myself but never have.
A good place to start is to try and dissect the example. Go through the code and try to understand what is happening and how it’s structured. It will feel overwhelming no doubt, but that’s normal and not something to worry about. Start changing some variables and see how it affects the function of the sample.
All of these things are relatively trivial and there are loads of ways to achieve them. That is not to say that it will be easy for a beginner or that it won’t take lots of practice and trial and error. What way you choose depends on your specific situation, personal preference, etc. Use the Defold manuals and the Programming in Lua resource, which is free. Read up on concepts like tables, which will be crucial for your storage system, and math.random() which will be the starting point for generating random encounters.
Of course. Either you will make your own map system, or you will build off the sample you mentioned.
If you are brand new to this, then I strongly recommend parking your dream Pokemon game until you have acquired some skills. The way I learn best is to make things, so I would personally suggest that you make tiny games to develop your skills, for instance by cloning classics like Pong or Asteroids.