THE USE OF DO TO AVOID REPETITION OF A PREVIOUS VERB

The auxiliary DO is used in different tenses and forms (did, does, didn't) to avoid repeating the verb used in the previous sentence.
Mother: Who made the cake?
Daughter: I did, Mom.
"I did, Mom." - instead of - "I made the cake, Mom."

1-Mary travels 10 hours a day. I don't know how she does it.
2-Most of the children came yesterday; just a few didn't.
In the fist case, DO is used as a full verb with the sense of performing an activity or task.
In the second case, DO is used as an auxiliary verb.
In both cases, do is used to avoid repetition of a previus verb.

DO is never an auxiliary when it's immediately followed by an object.
Don't forget to look over your class notes, Paul.
I've already done it, Mom.