Is Unreal open source?

[LogierJan]

Open source is a licensing model. Unreal is not open source.

meaning of open source:

Open-source license

Unreal Licensing agreement:

Unreal Engine | Publishing EULA

What unreal offers is access to the source code. This allows you to look into the engine code, and build upon it/extending it your to your own liking.

A Plugin is a module with reusable logic throughout multiple projects, if you’d want to extend the engine you’d put your logic in one of these.

more info on unreal plugins:

Plugins