App engine Instances placement

Whenever we provision an app in app engine flexible, where does google place these instances? In the documentation it only mentions that google manages these instances, but there is no mention of placement of these servers and how they are connected to a GCP project?

Google does not publicly share where this instances are placed due to privacy and security reasons. The instances are managed by Google, they are very well protected. The whole point of App Engine is having your instances managed by Google so you don't have to do it yourself, how they do it should not be a concern. If you are looking to manage your own instances, take a look at Compute Engine.