Common way to download open source projects
How many different way there is download open source projects in order to modify and rebuild them (directly in editors like eclipse and maven).
The short answer is "a lot".
Seriously it depends on the project. If for example project is developed by individual and not published at any public repository and exists on his hard drive only you do not even have a chance to know that the project exists.
If however the project is published in public repository it depends on the repository. The most popular way to develop open source projects these days is GitHub. If project is there you can always clone the repository and do what you want. Building the project depends on its author. If he/she uses "standard" build tools (maven, gradle etc) it is not a problem. Otherwise it depends...
If you do not want to modify the project but just want to include it as a dependency of your project it depends on the programming language. Java libraries are typically available in maven repository but it is up to the library author.
Please do not hesitate to ask more concrete questions.
I am a little confused on your question but it sounds like what ways there are to download projects.
Usually finding an SVN or GIT plugin would do the trick if you want to download it directly to the editor.