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.
Almost every company tries to force non-competition clauses in agreements. Even if they let one do side gigs, one can't work for competitor. This does sound reasonable and yet is a limitation on what you can do in your spare time.
Is it legal for something to be unfair against normal humans and in the favour of big companies? Tech companies spend 500,000 times your annual salary[1] lobbying the government each year, and you and I don't.
In some states, it is not legal for employers to take ownership of employees work off hours using their own equipment. Most employers still sneak these into their employment agreements even though they can't be enforced.
I cant say for sure, but I'd imagine so. As long as what you are doing isn't protected by some law, I imagine a company can discipline / fire you for whatever they please.
Yes, due to certain provisions in California Labor Code which allow for curtailing what you can do in fields that compete with your employer (in a large company, this generally means anything could fall under this).