AWS VPCs make it possible to establish private network connections across AWS accounts with VPC Peering, essentially merging the networks into one. However, there is another option for cross-account/cross-VPC network access, with a much smaller surface area.
AWS Availability Zone names may look like unique identifiers, but they are mapped to physical availability zones essentially at random. This means that us-west-2b in one account may be the same physical availability zone as us-west-2a in another account.