Backup Lifecycle & data flow
Workers, servers, how backups are processed
How does data transfer work between server and storage?
This all depends on your storage and server provider as they all apply different pricing and rules.
Some providers charge traffic leaving their infrastructure (called egress) and some charge entering traffic (ingress).In most ingress data is free.
What about backing up data from a DigitalOcean Droplet to a DigitalOcean Spaces?
"Traffic from Droplets to Spaces does not count against your Spaces transfer allowance (because inbound bandwidth to Spaces is free), but does currently count against your Droplets’ outbound transfer allowance. We’re working on changing this behavior."
So inbound traffic to DO Spaces is free, but outbound from DO Droplets may incur costs.