Once you know the frequency one option is to just take a raw sample at ____megahertz and play it back on demand. This doesn't work for some radio signals because they use rolling codes and it's also a bit inefficient (be VERY VERY careful using a Flipper with a car key fob, because they can sometimes go out of sync and you can't open your car afterwards)
The good news is, for many types of radio signals, the flipper can also determine the protocol and what digital data is being sent- so instead of playing back a 2 second sample of me holding down the "power" button on my lamp's remote, it knows it can just broadcast 0x1234 using protocol XYZ.
NFC and RFID devices are basically plug & play, although only a subset are supposed.
FYI many cars with "keyless" entry have a traditional keyhole hidden under a piece of trim around the door handle and a key (sometimes plastic) hidden inside the fob; sometimes the key is part of the ring for a keyring, and can be released by pressing on the manufacturer's logo or inserting a paperclip in a hole.
(Except for that suspiciously cheap gadget you got from AliExpress which shows up in the FCC database as an iPhone 4S...)
Remote door controls are painfully dumb and relied on the absence of affordable software-defined receivers and especially transmitters. With most of them you can set the code via binary DIP switches at the back and that's it. No replay protection, no nothing, if you're lucky the receiver has a brute-force detection.