How to fill multiple text fields automatically when the main input text receives a value and loses focus?

Question

I have a comic with the tables books and loans. On the table loans, Among other columns, I have one to put the title of the book and the code of this book. The idea is that, by inserting the code of the book and taking the focus of this input, the subsequent field is already filled with the title of the book in question (in this case, both the code and the title come from the table books).

I even get a certain result when I put a static value to test, but even researching materials on how to do this dynamically, I still can’t understand the logic behind this question.

By the way, I used the word "multiple" in the title, because I intend to apply the same reasoning of this question to do the same with other inputs present on the page I am working on.

How to proceed in this situation?

<?php 
$beta = mysqli_query($db, "SELECT titulo FROM livros WHERE codigo_livro='3'"); 

        if (count($beta) == 1 ) {
            $b = mysqli_fetch_array($beta);
            $titulo_livroB = $b['titulo'];
        }

?>

    <input required type="number" name="codigo_livro" placeholder="Insira o código do livro..." value="">
    <input required type="text" name="titulo_livro" placeholder="A ser preenchido automaticamente de acordo com o valor do campo acima." value="<?php echo $titulo_livroB; ?>">