They are randomly placed on the map. But remember that the strongholds need to be spawned at a new place where you have never been before. If you already had a big world before the 1.8 release, you have to go very far to find one. If that's the case and you want to find a stronghold, I recommend you make a new world.
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Yes
I would not assume so.
You have to find an End-Portal in a stronghold. The probablility of finding a stronghold with an End-Portal in it is small. Because there are 3 strongholds per map, and there is 1 end-portal per game.
A Minecraft world seed is what Minecraft uses to generate the terrain to begin with, it does not create any user-created structures like castles, only things like NPC villages, mines and strongholds.
Ender Portals are unbuildable in Singleplayer, but if you go into creative mode, you'll find the End Portal Frames and Ender Eyes in one of the tabs, the only way to find them in Singleplayer survival is to use Ender Eyes to follow and find the location of the Strongholds of the Portals.