How does a Debt Management Plan (DMP) work?

Looking into Debt Management Plans and trying to understand how they work from start to finish.

From what I’ve gathered, a DMP usually involves:

• One monthly payment through an agency
• Reduced interest rates
• Structured payoff over time

I want to know more details like:

• Does the nonprofit work/perform on the interest rate reduction before my first payment?
• What creditors work with DMP?
• Are there loans that can be helped with and in what way?
• Are you able to leave certain accounts out of the plan?

Would appreciate any real experiences or insights from people working as a nonprofit credit counselor.