I had a research article posted in arXiv on November 2020 and it is currently in review by a leading journal, I call it J1. Today when I was browsing research papers, I found that a research paper has been published in a lower tier journal (let's call it J2) and it has one section with ideas that are very similar to mine. The paper published in J2 was sent for review on 1st Feb 2021 and it got accepted on 15th Aug, 2021 (per the dates given on the website of the journal).
The section which is similar to mine has results which form the main idea of the paper and I find that it has been allegedly stolen. I am amazed how my paper which is already on arXiv can get scooped like this. Obviously the wording has been changed by the authors but the main idea in the section is the same as mine.
I am confused what should I do now. How do I prove the plagiarism, whom should I reach, and how should I approach this? Is the fact that their results in Sec 1 are similar to mine and I have published it first on arXiV not a valid claim to prove that my work has been scooped?
I am in my PhD days and the author who has done this is an Assistant Professor.
Note: By "similar" I mean that the results obtained in Section 1 of the paper in J2 and the results obtained in my paper in J1 are the same. The other sections of the paper i.e. Sections 2 and 3 in J2 are different from mine. But the results in Sec. 1 are very important and form a base for the other sections of the both the papers in J1 and J2.