From https://www.isbn-international.org/content/what-isbn

since 1 January 2007 they now always consist of 13 digits. ISBNs are calculated using a specific mathematical formula and include a check digit to validate the number. (e.a)

Each ISBN consists of 5 elements with each section being separated by spaces or hyphens. Three of the five elements may be of varying length:

...
Check digit – this is always the final single digit that mathematically validates the rest of the number. It is calculated using a Modulus 10 system with alternate weights of 1 and 3.