Boys send messages to girls wishing you a happy birthday, and girls reply "thank you". What do you mean?

There are several levels of understanding for reference only.

First, because of your blessing, the girl is shallow grateful to you. This gratitude is from the heart.

Second, because girls have a good relationship with you, I am very moved to see your blessing.

Third, the girl looks forward to your blessing.

Fourth, girls' manners.

Verb (abbreviation for verb) unintentional thanks.

I hope it helps you.