Achar
Sub-processors
Achar runs on AWS. That is the whole list, with one exception, and both are named here so that the answer does not depend on somebody reading a contract.
Amazon Web Services
Everything that holds content runs in AWS, in the account and region a deployment names: DynamoDB for documents and asset metadata, S3 for asset bytes, CloudFront for delivery, Cognito for sign-in, Lambda for the API, CloudWatch for logs. A deployment's region is in its own configuration, and this site's environment is in us-east-1.
Only if the deployment enables it. Google sign-in hands Cognito an email address and a name; no content and no other account data reaches Google. A deployment with no Google client id configured creates no provider at all, which makes this the one sub-processor that can be switched off entirely.
What is not on the list
No analytics vendor inside the product. No error-tracking vendor receiving request bodies. The marketing site you are reading has no third-party scripts on it, which is most of why it loads as quickly as it does.
Thirty days' notice of a new sub-processor goes to the billing contact on every paid account.