'Does' is the form of 'do' used with he, she, or it in the present simple. Think of one person doing something: She does the dishes every night. With you, we, or they, it stays 'do'.
Does in action
She usually does the dishes after dinner, so the kitchen is clean before bed. Here does is the main verb, meaning she performs the task, night after night.
He usually does his homework in front of the television, so the subject is one person spoken about, and the sentence is in the present simple. The word does is the main verb here, meaning he performs or carries out the task, and it is the form that follows he or she in the present simple, just as she usually does the dishes after dinner shows the same pattern.
When not to use does
Does pairs with one person spoken about: he, she, or it. Do covers every other subject, including you, we, and they. That agreement, not any difference in meaning, is what separates the two forms.
A subject like everyone or the dog takes does: Does the dog need a walk? A plural subject like the dogs takes do.
With singular they, meaning one person whose gender is unknown or unstated, use do: Does Sam drive? Yes, they do. Saying they does is not natural in English.
- The main trap is agreement, not meaning. Swapping does for do only changes the form, never the event described: Does she sing? and Do they sing? describe the same act, just with different subjects.