Voice assistants like Siri and Alexa understand speech through a process involving speech recognition and natural language processing (NLP). Initially, audio input is captured via microphones and converted into a digital format. The speech recognition system analyzes this data, identifying phonemes and words. Following this, NLP techniques are used to interpret the meaning of the input, allowing the assistant to understand context and intent. Consequently, the voice assistant generates a suitable response, providing users with accurate information or performing tasks based on their requests.