Pulkovo Airport is the only airport in St. Petersburg that offers domestic and international flights. The airport serves about 200 domestic and international destinations of several dozens of airlines from around the world.
How to get
The airport is 15 km from the center of St. Petersburg, you can get to the city by bus or taxi.
By bus
https://www.pulkovoairport.ru/en/transport/bus/
By taxi
https://www.pulkovoairport.ru/en/transport/taxi/
By car
https://www.pulkovoairport.ru/en/transport/car/
Official site of the airport: https://www.pulkovoairport.ru/en/