Scrum events, Scrum Master job and Dev team

Who can be a member of a "dev team", and exactly what is a "dev team" in Scrum.

As of the November 2020 Scrum Guide, there is no more "dev team". As recently as the November 2017 Scrum Guide, the Scrum Guide contained a definition for "the Development Team". The November 2017 Scrum Guide changed this to "Developers". The definition remains generally the same, however. The people who are accountable for and committed to creating the Increment are considered "Developers".

Who is obligated to assist to the Scrum events and why, are the PO and Scrum Master essential part of it?

It varies by the event.

The Sprint Planning, the Sprint Review, and the Sprint Retrospective are events for the Scrum Team. The Scrum Master, Product Owner, and Developers participate in these events. The Daily Scrum is for the Developers, but they may ask the Scrum Master to coach the team on keeping the event productive and positive or ask the Scrum Master and/or Product Owner to attend and participate if they feel that it would be beneficial to inspecting progress and adapting the plans to achieve the Sprint Goal.

What is the final objective of a Scrum Master in the Scrum Events, is it mandatory to keep assisitng tho the events after the entire team know's Scrum and works fine with it.

The role of the Scrum Master doesn't end when the teams are "fine" with the events. Just because the team understands the events, their purposes, and techniques for effectively running them doesn't mean that there aren't ways to improve how the team works.