“I don't know how you do it.”
You filled in for your coworker while she was out of the office on vacation. You had so much work to do while she was away, and it was difficult for you. Now you have a greater respect for the job she does. You say this while telling her how hard your week was.
I don't know how you do it.
This is a phrase that expresses respect for someone who is able to do difficult things. People often use this phrase toward people who have difficult jobs, such as:
You might also say this toward someone who has a lot of children, or someone who wakes up early every morning to exercise.
When you pronounce this phrase, the stress is on the word "how".
By the way, this phrase is a much more natural way to show respect for someone than to directly say something like "I respect you."