There is a distinction between being able to access the source code, and a tool giving it to you without any context of the underlying license it is governed by.
Good teams still document these. Amazon is apparently a leading example where the documents are considered the source of truth.
However many organisations have limited technical leadership enforcing quality, and management layers not willing to invest in these assets.
Unfortunately some 'modern' practices are being interpreted to make the processes like Agile the most important thing, and reducing the importance of things like code/system quality and knowledge management.
The OP is incorrect, most of the large organisations he cites do not practice trunk-based development.
Google has a monorepo and what could describe as 'submit patch for code review'. Only certain engineers can approve reviews and trigger the patch being applied to the trunk.
Many of the others also have variations, but unlikely that any organisation with 1000+ engineers has a monorepo with all of those engineers directly committing to it.
Edgar, you might be interested to know there are significant problems with the big javascript files being downloaded on Linux for Office365, particularly for WordOnline.
We've done similar experiments and proven it to be again based on your own user-agent detection, since normally they will take 30-60 seconds to load, causing the UI to essentially lock up and fail, when faking the agent-user to something else will work around it.
Multiple attempts at contacting Microsoft support come back with its not supported.
Its the Web... pretty sure you can build things without breaking them for Linux.