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Well, that's a big black hole. Creating a "virtual assistant" will require a lot of AI & Machine Learning knowledge. If you want my advice, start with a simple chatbot (check YouTube for a few tutorials) and start expanding from there. You need to give the bot an ability to browse the internet searching for a certain question and answering it (requires hard-API knowledge). Start from scratch, this is a big project. Slip it to a few smaller ones. Make sure to learn A LOT of things related to AI and Machine Learning.
https://en.m.wikipedia.org/wiki/Virtual_assistant https://www.cleveroad.com/blog/how-to-create-virtual-assistant-apps-like-siri-and-google-assistant https://chatbotnewsdaily.com/which-are-the-best-programming-languages-to-build-an-ai-chatbot-39eccf703aec #you can also surf in google for more better results.