As far as I know DocumentDB is closed source. Btw: I can name zero OSS projects from Amazon and this is not the same for Microsoft (VScode, TS) and FB (React, Jest).

Firecracker, s2n, chalice, jsii.

On top of that AWS has done a lot of contributions upstream, particularly around the linux kernel, https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin..., (particularly the virtualisation stacks involved), some around OpenJDK at what looks like an increasing pace, and the like.

To be clear: not an AWS employee.

I know several employees contributing to the Linux kernel, and I know they're hiring more Rust contributors. You're right though, I think Amazon lags compared to MSFT, FB, and G. Even Netflix or IBM/RedHat, for that matter.

lots of links: https://aws.amazon.com/opensource/

Firecracker is a famous one that I can think of.

Fork of Google crosvm

For some companies there are some (very partial) statistics, for instance: https://www.openhub.net/orgs/Google