How can private investigators do what they do?

They can't legally do what would require a warrant, like trespassing or wiretapping.

Most PI reports are about them sitting and waiting for the subject to leave the house. It's pretty rare when they catch someone hauling lumber or even changing a tire.

Honestly, these days a background and social media check is far more useful/dangerous than surveillance, and it's a hell of a lot cheaper.

Public domain is mostly how it's done. Nothing illegal as long as it's public.

Problems come in when they violate private domain. Like in your house. Or tracking on your vehicle. Stuff you own. Heck even your pets, since you legally own them.

Don't forget, the chips used in pets are trackers.

They cannot break the law. Even though some do. Like putting tracking devices on someone's automobile. If they get caught they will find themselves in court and likely lose their license. Which means they cannot advertise or operate a private investigation business.

You can record anything that happens in a public place, and you can find a stunning amount of info online.
Could be in some cases it's just a matter of waiting for someone to do something stupid.😆

They can't invade your privacy illegally or harass you. So they're limited to publicly available information.

They got a Cracker - Jack License to creep.

If you catch any private investigator doing anything illegal you should report them, real life isn't like what's seen on tv or the movies, they have to be properly licensed and obey all laws like any other person.