Hello Salma,
I cannot provide a complete and concise answer to your detailed question, but I'll try to help.
First of all, the article that you cite in your point #2 was published well before the significant changes to the PMP exam and the move to PMBOK 7, so it needs to be read in that context.
As someone who prepared for and passed the PMP in Jan 2023, I'd say that the detailed information about the process groups, as now available in Process Groups: A Practice Guide (PG:APG), was not of fundamental importance to answering the questions in the exam that I sat. I've briefly seen the PMBOK 6 (didn't have my own copy for studying) and I used the PMBOK 7 in my preparation. I'd say that PG:APG is simply all of the process-related information that was in PMBOK 6 and that doesn't feature in PMBOK 7. That made the latter a far leaner publication but meant that there wasn't an in-print document available for those wanting to understand the processes. If you have no access to PMBOK 6 and you wish to develop a deep understanding of PMI's process groups as a project manager, PG:APG is very useful. If your goal is more straightforwardly to pass the PMP exam, it isn't essential.
As to what you should be studying, I'd suggest that absorbing the Agile Practice Guide's content and being familiar with PMBOK 7 and the ECO are the principal things to which to devote one's time. Alongside that, I believe that practicing answering all of the the questions available in the PM Prepcast Quizzes and Exam Simulator is the best use of one's time. (I'm unaffiliated with the organization; I simply found the platform very useful in succeeding in passing the real exam.) I hope that others who have taken the exam recently will provide their perspectives to assist you.
All the best with your continuing studies.
