When I pick a bucket storage class in GCP (That's what I'm now using, but the question is also the same for AWS and Azure) I can choose the location that the data will be stored. This can be single region, multi-region etc and this means that when my data are stored, they will be stored securely in this region; also all data redundancy/backups etc take place in this region.
Now the information that I cannot find anywhere is what happens to the data during transit. Is it guaranteed that the data will stay in this region? If I pick US, do I know that while transfering my data, the data will stay in the US?
I do not refer to me being outside US while sending data to the bucket; I understand that this is physically not possible. The scenario is that a person geographically located in the same region as the region of his bucket transfers data to the bucket. Is there a guarantee that the data will stay in that region?
So about storing data at rest docs are fairly clear on that - your data will be stored only in region you specified:
Google Cloud Storage may be configured for data location pursuant to the 'Data Location' Section of the General Terms at https://cloud.google.com/terms/service-terms:
Data Location. For any Service listed at https://cloud.google.com/terms/data-residency, Customer may select a specific Region or Multi-Region as detailed in the Cloud Locations Page, and Google will store Customer Data for that Service at rest only within the selected Region or Multi-Region.
About routing your data in-transit it's different story. Normally you would use global endpoints to access your bucket and as name suggest they can be terminated in any region. There is feature called regional endpoint that ensures that your request will be terminated only in selected region but its in preview state and with very limited support.