I want some generic peices like corners and walls for building a generic fort. Are there any in game that work the best for custom building a fort?

There's no exterior set quite like that, but you can make do with simple fort pieces available.

What is the best way to find the fort pieces using the filter?


Also, how do I start building an interior. And then how do I connect it with the outside world? say a home in a city or a home somewhere? A door as an entrance.

go to creationkit.com. In the Tutorial section under Visual Learner you get a complete official Bethesda YouTube series that covers building a dungeon step by step. Very useful for beginners. They have text versions of the tutorials as well for quick look up of specific points. There are a lot of non Bethesda tutorials as well that have been very helpful to me. Watch those and you will get the basics of creating any interior and it's connection to outside.