Video from Bishop Robert Barron
"In fact, the 16th century was coming to an end when Giovanni de' Vecchi and Cesare Nebbia were commissioned to decorate the pediments of the great dome, with the representation in tondos of the four Evangelists: St. John (with the symbol of the eagle), St. Luke (calf), St. Mark (lion) and St. Matthew (angel); the contiguous upper triangles are decorated with angels designed by Cesare Roncalli.
This was followed by the decoration of the large dome, divided by 16 vaulting ribs which, within six fields, enclose: at the bottom, the Pope Saints buried in St. Peter's and the large figures of Christ, the Virgin, St. Paul the Baptist and the 12 Apostles; above, inside frames, angels with instruments of passion and heads of cherubs inside round medallions, followed by angels in adoration within frames and finally heads of Seraphims.
The top border bears the words S. PETRI GLORIAE SIXTUS PP. V.A. MDXC PONTIF. V., which was inscribed according to the wishes of Clement VIII in memory of the promoter of such a great work. Above, this is connected to the clerestory terminating in a small dome that glorifies the Eternal Father.
The iconographic layout, in which the possibility of viewing the details does not affect the overall harmonious vision, was executed by Cavalier d'Arpino who created 65 life-size cartoons painted with tempera, 16 of which were preserved in the Abbey of Montecassino."
from the article: The Dome - Interior