Some tips for Istanbul: https://hamdi.com.tr/ -> A not so fancy restaurant, locals go but really good food.

https://maps.app.goo.gl/AMQXZSN1dbd9EZaMA -> Tarihi SultanAhmet Koftecisi -> This is my favorite hole in the wall restaurant for Turkish meatballs. Best meatballs IMHO :) )

https://maps.app.goo.gl/bMYWkK9CKeWQb9gP7 -> Best baklava (and similar desserts) in Istanbul (or IMHO all of Turkey outside of the city Gaziantep) 

https://www.lacivertrestaurant.com/tr/hakkimizda -> Seafood on Bosphorous, great views.

http://www.kasibeyaz.com.tr/kasibeyaz-bosphorus -> This one would need reservation, great Turkish kebabs.

https://www.sevenhillsrestaurant.com/Main-Courses.html -> Another fish restaurant 

For fish definitely try the Turkish specialty fish called Istavrit. It is native to Istanbul, and you can not find it anywhere else outside of Turkey. (Well in Berlin, they claimed that they had it but it was too big and not good tasting) It is fried fish, and you are supposed to eat the whole fish (small fish) including the head (I mean at least I recommend you try one) 

Here is a cool neighborhood called Balat for Cafes etc. https://goturkiye.com/blog/hidden-istanbul-a-day-in-balat

You should also definitely try Istanbul Simit (which is thin bagel with sesame seeds) They sell it everywhere, especially by the boats in Eminonu and Karakoy. (Boats are called Vapur in Turkish). Also recommend you hop onto one and cross the Bosphorous. You can have cay (tea) and simit and enjoy the sea views (my favorite thing to do in Istanbul). There is also a famous simit shop but I think it is a bit overrated, the simits on the streets are even better. https://maps.app.goo.gl/TY4T5ejiQJh1Ue7t8

By Vapur, I mean these boats. Those are the authentic ones. There are also ferries, but they are not fun. There is a vapur that also goes along the whole Bosphorous but that one takes a long time. If you have time, that is also very nice and when you reach the other end of Bhosphorous (called Kavak) you can eat at one of the authentic restaurants by the water. https://www.sehirhatlari.istanbul/tr/seferler/bogaz-turlari/uzun-bogaz-turu-91

One more food tip: Kumpir -> This is baked potato stuffed with all sorts of things of your choosing. The choice of ingredients in Istanbul makes it different than any other baked potato. And also the place where you find Kumpir places is a very nice one, right next to a famous Mosque called Ortakoy. https://yolcu360.com/blog/ortakoy-kumpircileri/