Every time I press a function key on my remote an annoying robot voice assistance come over my smart TV explaining what I pressed; and, as I toggle through movie selections, this same annoying female voice describes in detail what the movie is about. How can I deactivate this feature?
It looks like you may have accidentally enabled "Audio Guide", a mode for sight-impaired viewers. That's why your Roku player sounds like it's been talking to you. You can turn off Audio Guide by pressing the * button on your remote 4 times quickly.
Regarding the Roku Express, you have probably enabled/toggled the Audio Guide (this is the first voice you are hearing explaining what you pressed/navigation/reading menus etc.) Press your *button 4 times to enable or disable this feature. https://support.roku.com/article/231584647
Regarding the 2nd voice discussing narration of what is occuring during movie, this is normally an Audio Description feature inherent in some channels. When you start a movie, press the *button once, and select a different Audio track. (one that does not include Audio descriptions). Now sometimes, your TV itself may have additional settings regarding Audio Descriptions, or you may have to select Stereo Only under Roku Audio Settings. Normally, just selecting a different Audio Track will work though.