How do FAANGs document their infrastructure?

This is a very broad question and it would help if you could follow up with your definition of infrastructure.

At Amazon, we would use a variety of solutions to document the build process, how to set up internal accounts, how to respond to pager calls, etc. Amazon has a custom documentation system, but it isn't a whole lot different than the various WIKIs that are available outside of Amazon. Some of what you see as documentation for AWS [1] are an evolution of internal documents from when the solution was used internally before being made public.

My time at Apple was earlier, but the systems were similar. Various internal web sites, files checked into cvs/svn/etc and assorted custom issue tracking tools.

I wish I could say there were amazing and magical tools that made documentation easy and painless, but there weren't during my time at Amazon and Apple.

There might be some very nice tools available that can traverse and document network topology and services that I wasn't exposed to. Hopefully someone will come along and let us know.

[1] https://docs.aws.amazon.com/index.html?nc2=h_ql_doc_do_v

Its a smorgasbord of git commits, READMEs, wikis, Jira tickets, documentation 'websites', word documents, google doc like documents, etc.

There is a bunch of information on how Google does documentation in the book Software Engineering at Google:

https://www.google.com/books/edition/_/WXTTDwAAQBAJ

One notable practice mentioned in the book is their g3doc system. Documentation is kept in the (google3) repo alongside the things documented. It is encouraged to update the documentation and the code at the same time. This also means the full history of the documentation is available in repo rather than in a separate wiki history.

There is also a system of URL-shortener-esque "go/ links" that allows people to easily link to or jump to documentation. For example, "look at go/foobar". The docs are also exposed in their code search system.