If you want birds to use a nestbox the direction is generally important because of prevailing weather conditions (which will depend on where you live).
No, it should be placed horizontally. If it on the top rain will get into in and drown the chicks. If its on the bottom nothing can get in once the nest is built.
They are creating bluebird trails. Bluebird trails are simply a line of bluebird houses. Some have only 4 houses and some have over 100. This gives many bluebirds a place to make a nest and lets the population grow.
Bluebird trails are lines of bluebird houses. There could be four house in a row or there could be hundreds. It gives many bluebirds a safe place to nest and lay eggs.
Not in one where the bluebirds already are nesting, but wrens are cavity nesters, and will nest in unoccupied bluebird houses.
Clean them once the bluebird abandons the nest; usually after the babies leave the nest. If a sparrow or other bird starts making a nest over the current bluebird nest remove the materials of the nest in process. Do not clean out the bluebird house if the bluebird is still using the nest.
As big as can be
Apaches live in the Southwest.
wood
For the Navajo, the houses they build are called hogans. For the Hopi, the houses are made out of adobe.
wooden bricks
long houses some in lakes and humans some lived under ground
They give many bluebirds a place to live at the same time. The population can increase because of the amount of bluebirds that can nest in that area.a bluebird trail is a group of nesting boxes. this helps the bluebirds because then they have places to live and have babies.