The Fortran 2023 standard is sold at the ISO web site for CHF 216. The standards committee uses its Interpretation Document as a reference. The main new features are resumed in its introduction, pages xiii-xiv.
John Reid wrote The new features of Fortran 2023 and co-authored Modern Fortran Explained: Incorporating Fortran 2023, 6th ed.(2023), Oxford University Press
Jacob Williams listed some of the new features and his opinions of them in a post The New Features of Fortran 202x