From
present perfect. Action has been finished.
Transitivity
Using 〜てある with an intransitive verb does not make sense.
If the verb is transitive, the verb remains a transitive verb, but the focus can shift to the object.
を→が
"Who does it" (original が) drops out.
橋を開く I Open the bridge (transitive form of verb)
橋が開いてある The bridge has been opened (transitive form of verb)
The てある form of a transitive verb can use either the が or the を particle, depending on the focus. が puts focus on the thing that is made, whereas を puts focus on the maker.
寿司とサラダを作ってあります。 I made sushi and salad.
寿司とサラダが作ってあります。 Sushi and salad are made.