Hi, I make some comments on 2 and 3. I have no prior experience with the Oauth standard.

Regarding the management of users, individual privileges, groups or organizational divisions can be given for visualization and participation in processes (execution of tasks). All of this is perfectly possible and (relatively) easy to administer.

On the other hand, in terms of the logic of the processes, there are a large number of moments in which code can be included. in particular, so-called "tools" can be included within the processes that can execute Java code to carry out very specific tasks.

But for both, if you have not done it before, I would recommend running the tutorials (https://www.joget.org/tutorials/). In several of them allusion is made to these issues.

Q1, need a OAuth plugin that fits into plugin category "Directory Manager" - Directory Manager

Q2, see user setup at Managing Users. User setup and app design are 2 distinctive things. You used "project" term, I guess it refers to an entity you have inside a Joget app. Imagine "project" is part of an approval process, we will then perform user assignment to work on it accordingly.

Try out the video tutorials or run through the sample "expenses claim app" in default Joget installation then you will have better idea on how to work things out.