How to Make the Layout Development of a Eccentric Reducer and Concentric Reducer

If it is for pipe, this is one of the classic sources:
The Pipe Fitter's and Pipe Welder's Handbook, Revised Edition by Thomas W. Frankland

For sheet metal drafting, the topic is often called "developments and intersections".

This is hard to explain in words only, so it is best to consult a textbook.