What happened to Zoroastrianism after the fall of Persia? Is it still worshipped by even a small group of people? What are the fundemental principles?

I can answer the second question most easily: yes, it is still practiced by a small and shrinking group of people, primarily in India. The largest community (~60,000) is the Parsi community in India who left Iran (the traditional Zoroastrian homeland) roughly a millennia ago. The next largest community is still in Iran and there may be a few other remaining communities in historic Greater Iran, but I'm not sure about this. They may have all left at this point. There is also another significant South Asian community, the Irani community, who left Iran in the 19th century (the 60,000 number for Parsis almost certainly includes Iranis, who only number a few thousand). Both the South Asian communities and the Iran one exist in diaspora, particularly in English-speaking countries like the US, UK, and Canada. The Parsi community is particularly rich and notable, as they functioned as a middleman minority especially under the British Raj. Harper's Magazine recently wrote a long and very interesting article about the Parsis in Mumbai (formerly known as Bombay). The article is gated, but maybe you can track it down in a library. The BBC wrote a similar article a month before, which conveys most of the same information though I remember liking the Harper's one better (if I remember, they dealt with the current religious conflicts within the community in more detail).