Hi, glad you looked at the tutorials. Here's the complete list for context https://github.com/ignitionrobotics/ign-physics/tree/main/tutorials

1. To answer your first question, the physics plugin is only an interface to physics engine and cannot modify physics engine behavior. To do that, you will need to modify the particular physics engine source code.
2. To see how to link custom physics engine to ignition, you can follow the tutorial here https://github.com/ignitionrobotics/ign-physics/blob/main/tutorials/09-set-up-physics-engine-tpe.md. Basically you will need to implement a physics engine API, and set it up following the example of TPE https://github.com/ignitionrobotics/ign-physics/tree/main/tpe, then write physics plugin feature for the physics engine.
3. In the tutorial linked above, there is a minimum example focusing on EntityManagement feature in physics plugin and implementation in TPE.

Hope this helps

This is a big question! If you're willing to build the underlying physics engines from source, I think you can change just about anything. Here's a list of approaches you can take for osrf/gazebo (I will leave discussion of ignition for a follow-up post):

1. To add functionality using the gazebo::physics APIs that are agnostic of physics engine, you can write a ModelPlugin or WorldPlugin. See the HarnessPlugin for example.

2. You can also write plugins that are specific to certain physics engines if the gazebo::physics API is not enough. For example, the WheelSlipPlugin uses ODE slip parameters that aren't broadly exposed by the gazebo::physics API.

3. To change the way Gazebo interfaces its gazebo::physics APIs with a physics engine, modify the files in the sub-folders like gazebo/physics/dart or gazebo/physics/bullet. For example, see some changes to support of kinematic loops in DART in bitbucket pull request 3086 merged in osrf/gazebo@bee69082.

4. To modify the way a physics engine works, you can build it from source. The easiest example of this is by modifying our fork of Open Dynamics Engine in the deps/opende folder. For example see a modification of the Gearbox joint in our ODE fork in bitbucket pull request 3073 merged in osrf/gazebo@e11215a7. There are some old instructions for building the other physics engines from source using a catkin workspace, though I would now recommend using a colcon workspace.

5. Adding or modifying the gazebo::physics API that is agnostic of physics engine is a bigger endeavor since it may require modifying the interfaces to all the supported physics engines.