How is Ukraine able to strike deep within Russian territory?

The governor of Russia's Kursk region says a drone has attacked an airfield setting an oil storage tank on fire. The region borders Ukraine, and the incident comes a day after multiple explosions at two military bases deeper inside Russian territory. Moscow blamed Ukrainian drones for those blasts. Kyiv has not claimed responsibility.
On Monday, Russia launched another wave of strikes against Ukrainian cities. President Volodymyr Zelenskyy says missiles killed at least four people and knocked out power in many parts. Ukrainian officials are warning several regions are now facing more emergency blackouts.