

I would like to point out that they are free to modify the source code before building the binary they distribute. Being open source does not mean protection from secretly stealing data.
With chrome it is obvious because the closed part is called chrome and the open is chromium. But it is certainly possible to not make “stealing” magic on top public.
Most code I forget too. For a lot of stuff I might vaguely remember when looking at it but some I would deny ever even having seen even when I wrote it (that happened). However, there are some rare parts I could probably still navigate from my previous company and there are some in my current. Really depends on the connection you have to the code.