This is the 4103rd most frequent Russian word.
питере
"Питере" is the prepositional case of "Питер," a casual nickname for Saint Petersburg, meaning "in Saint Petersburg."
In this sentence, 'питере' is the prepositional case form of 'Питер', which is a colloquial term for 'St. Petersburg'. It's used after 'по' to indicate the location of walking.
Мне нравится гулять по Питере солнечными днями.
I like walking around St. Petersburg on sunny days.
Here, 'питере' is again the prepositional case of 'Питер', used with 'в' to express location, stating where these bridges are found.
В Питере очень много красивых мостов.
There are many beautiful bridges in St. Petersburg.
In this question, 'питере' is used to inquire about the location pertaining to the verb 'быть'. It is commonly used after 'в' for establishing the presence of someone or something at a place.