Novice in speech recognition

Hello my dear, my name is Thiago, I am a master's student in computer science, currently focusing on software engineering. However, in the past few weeks, I have been studying machine learning because I am participating in a project that aims, among various objectives, to implement voice recognition for Android devices aimed at urban environments, and possibly for the armed forces. I have some knowledge in signal processing, but very little about semantics, formal languages, and NLP. I also noticed that there is little material available as an introduction for someone interested in delving into this topic. That said, would you mind listing the contents and topics that I should learn and master before starting to implement this type of tool? My final idea is to use a pre-trained model and perform additional training in PT-BR to increase efficiency given the language and context. I appreciate any help you can provide.