زَيْدٌ حَاضِرٌ، وَهُوَ مُجْتَهِدٌ.
Huwa refers back to the named Zayd: an ordinary third-person pronoun.
هُوَ اللَّهُ أَحَدٌ.
In the book’s analysis, huwa introduces the affair explained by the following whole sentence.
Distinguish an earlier named referent from a proposition explained afterward.