What you need is called Wake Word and Phrase Recognition. But you can be sure that it is much more difficult to achieve than it seems! You'd probably have to consider an STM32L4 or even more powerful device to perform this.
You will find companies in our partner program providing solutions for that approach: https://www.st.com/content/st_com/en/partner/partner-program.html?key=voice#2
Good luck,
/Peter
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