By default it's not. However, if you join FAANG, chances are that you'll be required to sign a contract that basically says "I waive my rights of doing side-pro

By default it's not. However, if you join FAANG, chances are that you'll be required to sign a contract that basically says "I waive my rights of doing side-projects to to the greatest possible extents under the laws of California". When speaking of contributing to open source, Google generally has a more positive image, but there are still occasions that something could only be released after being reviewed by the legal department if it's work-related. On the other hand, Apple is known to have strict restrictions.