Your residential is a requirement to finish your Gold DofE. It is not part of any other levels. Residential involves spending 5 days (4 nights) away from home on a shared activity with people you have never met before. To clarify, this is an individual section - you won't be going with people you know (The only exclusion to this being if you are going to a large residential location with many people).
In your Residential, you should take on an activity that requires you to work both in the day and in the evening. Both times should be just as much part of the experience. Whilst there, you should be given the appropriate training and you should find an assessor.
As for what it can be, it can be anything as long as you are learning something whilst you are there. The Residential section is very flexible as to what is accepted. As well as this, you must find a place where you can stay on-site.
If the camp/jamboree goes on for a minimum of 5 days (4 nights) then you can use this as your Expedition.
Work is always required at a camp/jamboree so, if you can volunteer to help run one of the variety of activities there then this is a great option to take!
Learn a language abroad
Interested in learning a different language? Learn better by doing? Why not try learning a language in your desired country?
This doesn't just involve going around trying to understand the language, you should also get involved in activities whilst you are there. It's a great way to learn and, you may even learn additional skills beyond the language!
Community building is a fantastic idea for your Residential. If you can help out in a community that is lesser-built communities get what they need then not only does it look fantastic on your CV but, it is also an excellent experience.
Some examples of what you could be doing include (re)building schools, medical centres any anything else that would help the community.