How do successful tech companies write and manage their software specifications?

How do successful tech companies (e.g. Apple, Google, Microsoft, <add your favourite>) write and manage their software specifications? I.e. what kind of tools do they use? How do they keep things up to date? How do they write down their tests?

Background:

I see many small companies writing software specifications in Excel. One file for requirements, one for functions, one for tests, one for GUI screen. … And variations of that. This is a nightmare to work with and absolutely not scalable. Also, connecting specification and auto tests is often done manually – also a nightmare.