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Regarding the reason why the question was closed, ignoring the fluff/rambling, there are at least more than 1 tangential questions:
- Change the audio stream's volume for the ringer, media, notification, and assistant (Android provides many different audio streams, but it is up to the manufacturer/ROM on how to manage them)
- Adjust the Assistant's volume to the user's speech volume and customize its behavior on non-verbal activation (personally, I'm not sure if this is even possible at all)
- Silencing/muting the phone without looking at the screen (there are alternative solutions like automation and "do not disturb" mode)
While all of them are related to audio volume, solutions to one issue are possibly unrelated to other issues, making it unfocused (previously known as "too broad - multiple questions on a single post").
The problem with multiple questions on a single post is, answerers are expected to solve all the issues, preventing them from posting an answer if they only know the solution to an issue or two. Evaluating the answers also becomes difficult since an answer may contain a good solution to one issue but poor solution to another, while another answer provides vice-verse, making it a confusing situation for the voters and future readers.
Take note that developer-related questions (like asking for the API and how to build a ROM) are generally discouraged since the user-base of this community is Android end/power-users, but not necessarily developers.
The fix is to separate these issues into multiple posts, one focusing on a specific topic. Before posting a new question, please also search for existing questions to prevent duplicates.