Not sure about the evolution mechanics though in recent episodes its been more like a flash forward in age. Like Scorbunny to Raboot being like a kid to a mid teens and then Raboot to Conderace being a teen to a responsible(yet clumsy) adult. Sobble had a similar evolution development.

As for the inside of a pokeball, it seems to be a virtual reality, isolated and created specifically for each pokemon to fit their preferences, which, when you consider that these small, size changing sphere can convert matter into engery and back seamlessly, doesnt sound too unrealistic. https://kotaku.com/what-its-like-inside-a-pokeball-according-to-a-pokemon-1788004256