[giaour]

> Honest question - how do you actually effectively share code between many repos?

Locally, you can use an Amazon-internal tool to check out multiple repos and make changes to all of them. The tooling calls this a "workspace," but it feels very much like working in a monorepo since building and testing can happen at the workspace level.

> How do I know that me changing my backend app’s API doesn’t break any other project in the company potentially calling it?

In terms of change management, Amazon dependency graphs are managed as "version sets." Changes have to be built into a given version set, and that build will also rebuild any package in the version set that consumes the repository whose changes are being built in. (Usually, repositories are configured to build into one of the owning team's version sets on each commit to the primary branch.)