the popup from Amboss gives this info A peripheral nerve in the upper limb that arises as a continuation of the posterior cord of the brachial plexus (nerve roots: C5-8, T1). The radial nerve innervates the triceps, brachioradialis, supinator, and muscles in the extensor compartment of the forearm, as well as the skin on the dorsal surface of the arm, forearm, and hand.
https://next.amboss.com/us/article/OJ0I8S?utm_campaign=EN_XX_Browser-Extension-Chrome&utm_medium=browser_extension&utm_term=Radial_nerve&utm_source=app.ankihub.net&m=xTaEGP#Zaf4ce6b79c9cf963843b0a20382649a3
this picture from amboss also illustrates only dorsum of forearm