Since this thread was bumped back up from over a year ago, I’ll just answer this question.
Blender and Godot are Open Source because their source code is available for anyone to use however they’d like, in addition to being able to contribute back to the program. In Blender’s case, if you use their code, then your application also has to be Open Source.
The reason Unreal Engine is not Open Source is because its source code is not available for anyone to use. It’s simply available as a reference for developers, and like Open Source, it allows the community to contribute code back to Unreal Engine. Even if you do use the UE source and make your own program out of it, Epic Games still has all the rights to it.
When this is the case, where you can find the source code for a program but can’t use it however you’d like, it’s called source available.
There is a huge misconception that if a program’s source code is available, that it is Open Source. This is not the case. As @LogierJan said, Open Source is a licensing model.
Plugins have nothing to do with the source code subject. They are a way to extend a program and add functionality into it that wasn’t there before. Most modern-day programs support this, including Photoshop, Unity, and Google Docs, even though none of those programs are Open Source or even source available. Unreal Engine supports this concept as well.
For more detailed information, reference the post in this thread by @LogierJan.