How to explain a physics mechanism that I study from other paper in my report while avoiding plagiarism?
You can quote (within reason) and/or paraphrase quite freely as long as you cite the source. For some things there are only a very limited number of ways to state something accurately so "using the same words" is not an absolute prohibition in certain fields. There are scales of measurement here. But in those situations in which you need the exact words (such as, perhaps, definitions), quote and cite. 

Plagiarism is when you try to represent another's work as your own. You aren't doing that here. Copyright infringement is when you copy another's words without permission or other justification. Normally, academics expect that short phrases may be quoted from their work. For longer passages, however, it is wise to seek permission from the copyright holder, of course. 

However, there may be another way in some circumstances. Sometimes it is possible to take older work and summarize it in a way that loses almost nothing. Rephrasing work in a simpler form may help both you and your reader to understand it better than the original sources. It isn't always possible, but may be worth the effort. This takes insight, which is worth cultivating.