Your baby uses 'prosody cues'
Each language has its own pattern of intonation, stress and rhythm; this is known as 'prosody'. Your baby has been able to identify different languages since they were born using ‘prosody cues,’ so by listening to how things are said, and particularly by listening to intonation, your baby is able to identify if the person speaking is using a language that they are familiar with.[1]
Sharing nursery rhymes is a perfect way to embrace intonation and rhythm with your baby. Use lots of different tones, vary your volume and follow the rhythm of the rhyme as you sing or say the words.
References:
[1] Chong, A. J., Vicenik, C. & Sundara, M. (2018) ‘Intonation plays a role in language discrimination by infants’. Infancy 23, 795–819.