“The concert was great. By the way, I saw your cousin at the store. I didn’t know she was in town.”
“This author is very famous. I’ve read all his books, by the way.”
“By the way” is a phrase we use to introduce a new topic (“I saw your cousin”) or add some additional information about the current topic (“I’ve read all his books”).
「コンサートは楽しかったよ。ところで、君の従妹を店で見かけたけど、帰っていたの？」
「この作家はとても有名。余談だけど、彼の本は全部読んでいるよ。」
「By the way」はカジュアルに新しい話題を出す（従妹を見かけた）、又は今の話題に情報を追加するために使うフレーズです。
