It seems the FBI never got a warrant, when they sent a spy in the Trump campaign. Wouldn't this be required?

No. Informants are often people who already work on the campaign and are just asked to keep their eyes open. Back in the day, in the 1960s and 1970s, they had as many as 15,000 such informants watching left wing and right wing fringe groups. We may have that number now, mostly watching Muslim groups. They are supposed to be reporting on federal crimes, not influencing the groups direction (though the COINTEL program was successfully sued for some informants' excesses).

An FBI agent I knew, once told me that if the FBI ever stopped paying their informants, there would be no Communist Party USA. And sure enough, when they did stop paying them in the 1990s, the CPUSA virtually ceased to exist and is now sort of Social Democratic.

That s only if you believe Trump.

You need a warrant to make an arrest, but not to investigate things. The FBI uses agents provacateurs all the time.

The purpose of the FB"I" is to investigate. The government already knows what they are there for. A warrant is not needed to investigate.

They are Gaslighting you the FBI was being the FBI. How do you know what they did?

There was no 'spy' in the Trump camp. Also, no a warrant is not needed if the FBI is attempting to gather intel regarding possible crimes.

No. A warrant is required (sometimes) to search without consent. Sending someone to observe is not a search.

As of yet there is no evidence that the FBI sent a spy in the Trump campaign.

No because an informant is someone Who is there to observe. He aleady had permission to be there so he doesn't need anything as far as paperwork

They sent an undercover investigator to see if indications of possible crimes had any evidence.. Got any evidence there was any political espionage?