As the title suggests, I'm curious what determines which word is used for "to be made."
For example, what are the nuances between
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作られた tends to be used when you have some "creator" in mind, whereas できた tends to be used when who made it is not important or something came into existence on its own. Please keep in mind that できる is fundamentally an intransitive verb although it's commonly translated into English using the passive voice. In your case, 法律が作られた tends to be chosen when you are interested in the law-making process, whereas 法律ができた tends to be chosen when you're interested only in the resultant fact.
To give other examples, 近所に郵便局ができた is usually more natural than 近所に郵便局が作られた because most people are not interested in who made the post office. 森ができた refers to a common natural phenomenon, but 森が作られた may sound like it's a man-made forest.