How do I make an app that generates answers from an uploaded text file automatically?

Hahaha, if you can do this, then you’ll have a multi million dollar product. However, it is do-able, theoretically….

Parsing – Parse, or split, out the sentences
Metadata enrichment – Attack the sentences with all sorts of NLP APIs, to enrich the corpus of sentences with various and sundry metadata
Question generation – Do question generation on the enriched dataset
Recommender system – Pair the questions with their most likely answers (sentences), into question-answer pairs
Machine learning – Feed the question-answer pairs into a neural network
Dialog system – Write an engine that can query the trained neural network in natural language
Bingo – You’ve got a multi million dollar product

For more detail, see my quick and dirty webpages: