Question: what do you do after you beat team rocket in mahogany town?
Answer: Head to the Mahogany store, and you will see a small cutscene leading to a secret stairway to a Team Rocket Hideout. Navigate and clear the Team Rocket hideout. Defeat the Mahogany Town gym leader. Return to Goldenrod City and defeat Team Rocket at the Radio Tower and at Goldenrod Underground.