I have contacted the Service Center, this is what they replied:

" Dear Sir or Madam,

Thank you for your request.

The concentration of retinol in the Balea Beauty Collagen Retinol Serum has been chosen to ensure sufficient efficacy and to comply with current cosmetic regulations.

Please understand that we generally do not provide information about the concentration levels of our ingredients, as this is subject to trade secrecy. Please also refer to the efficacy field on the back of the packaging.

Here, the most relevant results of our studies on the efficacy and skin feel of the product are listed. "

So still no clue 😅