I've been searching for the word, which is used for describing parts of lyrics, where the singer sings for example the whole refrain only using words of syllables without any meaning. Maybe you could help? (here's one song with that type of refrain: https://www.youtube.com/watch?v=zIgrwyLU3Ck)
Non-lexical vocables, which may be mixed with meaningful text, are a form of nonsense syllable used in a wide variety of music. A common English example would be "la la la", "na na na" or "da da da":
Lilting is a form of traditional singing common in the Goidelic speaking areas of Ireland and Scotland. [...] It in some ways resembles scat singing.
The example you give is partially in English and partially in what I undestand is Gaelic: I couldn't tell you if the refrain has words in it or not: the song Nil 'Na Lá is a traditional song, whose title means "Daybreak has not yet come". https://brendannolan.com/lyrics/nilnala.html
- Scat is the term used for jazz non-word singing Wikipedia
- Lilting is for Gaelic non-word singing Wikipedia
- Yodelling is the Alpine form of non-word singing Wikipedia
Wikipedia lists many other forms at Non-lexical vocables.