Hi Michael.
You meant for Fallout 3, not 4 right? With FO3, most of the DLC have a separate world space that you have to travel to in order to start them. There is a riverboat on the Potomac you board to travel to Point Lookout. There is an Metro Station you enter to do Operation Anchorage, etc... You can get sucked into the Mothership Zeta dlc simply by straying too near a crashed alien spacecraft, so that one can be a surprise if you are not ready for it. You would have to travel to all of these locations to start their dlcs.
The only real oddball is Broken Steel, which has some game tweaks that automatically take effect, like increasing the level cap to 30, and adding some new high level monsters. Otherwise the quests and areas it adds can only be accessed by finishing the vanilla main quest. BS basically extends that, and allows you to play the game after finishing the main quest.
This post has been edited by SubRosa: Jan 13 2017, 02:13 AM