How do you make a demo version of your game, trailers, Cutscenes and DLC for a game? I wanted to know how can and what tools is there for me or any developer on how to make it? im gearing up to study Game development next year in technical college. please help me and how hard or easy it is for what I explained in the title.
Demo version of the game? Just make a simpler or cut down version of the game in the editor. That’s how I would go about it.
Or create a specific enviroment and level just for the demo using the assets from the main game, like age of empires 1 did! (This is the best way for people who want more but different stuff.)
Trailers? Some gameplay with editing inside of a editing program like vegas or the like. Not to hard.
Cutscenes, you could create a in game scene inside the engine in real time for the player using camera’s and animations. Or you could do it in another program or UE4 and recored it and play it as a video file.
Or you could make a picture cutscene like in Thief the dark project. https://www.youtube.com/watch?v=fSET0AZfbyE Or just put text… or none at all! Not all games need them! 
(Or get inventive and do something new!)
DLC? There is some stuff on that here:
Patch and DLC support - Platform & Builds - Epic Developer Community Forums
But that is up to how good you are with the editor and possibly programming. Though you could just ask the question on the answerhub and get a more professional answer. As I have not much knowledge on this subject!
Hope some of that helps.
It really is up to how well you can use UE4 or whatever engine you use.
Now for tools to use! UE4 (You are on the UE4 forums…) Unity, Cryengine and many others. (Choose what engine suits you.)
I would also check out the content creation tab and look at the free resources stuff and see if any of those help for asset creation for the game engine.
You will also need to learn some programming (Or get a friend that Knows it.) either it be the simple to use (Well for me it is… Sometimes!) blueprint of UE4, or the complex C++ or the easy to learn python on unity. (Or whatever they use.)
There is many routes. But I would suggest checking out the UE4 documentation on Blueprints first.
Also, A word of advice. To be a game creator, you must also be a game player. (To an extant.)
Which I bet you are, but play some games and see what they did wrong and what they did right and take notes. (Especially well beloved games old and new.)
No better way to learn what makes a good game playing a bunch of good and bad games!