Five Eyes appears to grant the ability for the other countries within to spy on the others within.
There are no laws in the US that are effective against the UK, for instance, as their authority stops at the border. When you tell the UK they can spy with abandon on the US, except they have to give all that information to the US that it wants, you start to understand. An example would be to ask MI6 to spy on Donald Trump, giving all backdoor access to systems that the FBI and others have already, thus negating the need for the FBI to ever get a court order or subpoena to access data already available to them. MI6 would then use the system and give the FBI or others what it found.
Of course, it works in reverse. There is no UK law the prohibits the US from spying on the UK, so the UK gives US intelligence access to its systems and has full plausible deniability for everything US intelligence pulls from its systems, for the UK’s use.
It’s quite an interesting scam, with only citizens left at risk.
Authorities then learn about the things they want and only have to get court orders or subpoenas for the exact stuff it already has in hand from its Five Eyes partner.
That's what I would do if I were an evil despot.
That is a legal fiction and a criminal conspiracy to break US law. It is illegal for them to spy on us at home, so they arrange for someone else to do it for them. Five eyes was illegal as hell from the moment it started, and that is one of the main reasons it was secret.
It's a perfect way to subvert US laws against CIA spying on US citizens.