Are all Blueprints connected?

Are all blueprints within a project connected and influence each other?
As I recognized is the level blueprint always working.
If I create a new blueprint actor, will it run? Does it need to be connected to the level blueprint or are all blueprints working?

Blueprints are only connected if they share data (variables, structures, references etc), and you do not have to register-connect anything, unless you want it to be. One problem is when you delete a blueprint which is connected to something else, Unreal Engine has a hard time to update references, or to remove them entirely. If you force deletion of such a blueprint it can lead to crashes, long delays, or even brick a project. This can also happen if you move blueprints inside the project when a map containing related references is open.

Because of the poor file operations in Unreal Engine 4, I recommend to either manually wipe Blueprints by opening them and delete content prior to deleting them, or for instance when is known that a file is not connected to something else yet, close the editor and delete the file from the explorer, i.e. you can do this with maps of newly added content packages. Before you delete something also right click the related folder and use the Fix up Redirectors function, which otherwise can break stuff.