How to write a research proposal for a PhD position where the description of the project is given? I saw an offer for a PhD position where the title and the description of the project was given.

Now, in the form they ask the applicant to write a research proposal which will include 'Why I'm interested in the field/project described in the advertisement; 2. Why and how i wish you complete the project; 3. What makes me suitable for the project in question.?' (maximum 3 pages).

In my university, in one course what I learned was a complete idea how i will do the work and finish that work. But where should I write the reason why i m applying there? I mean, if they need a new proposal then would why they write the project name and description? And how is one supposed to know how the supervisor wants to complete the project?
If the research project description is very specific, think of it as a job application.

The research project is the job advert.

The research proposal you will write up and send out is your cover letter. You ought to demonstrate how you're the best fit for the research project.

Approaching the research supervisor in advance of the application deadline would be good, if their details are given as part of the project description.

One cannot assume that 'the supervisor knows how they want to complete the project'. One supervisor might be very prescriptive, the next supervisor will be very open-minded or loose.