What is the difference between muscles being “told what to
do" by the brain and being "monitored” by the brain?

There are voluntary and involuntary muscle movements, put your hand on a hot stove for example and your reflexes generally do it for you. That is involuntary and movement produced by the nerves and nerve ending because of the pain that your nerve felt. The doing after telling the brain is something called motor function and the movement isn’t produced on reflexes and nerves alone, it’s a combination of your muscular system and brains motor function working together.

Hope this helps!