For clarification about car keys and basing my answer on a Citroen C3 that i had to duplicate the fob …
Car keys in general have 3 components :
Key blade - that operates the cylinder and have to be duplicated by cutting a compatible blade with a machine or laser or whatever - this makes the cylinder rotate to open the doors and rotate the ignition switch
SubGhz Radio - Sends a rolling code for example on 433.92 Mhz to open and close the doors by radio like on a garage door, etc …
Hitag2 or similar chip - Inside the plastic key compartment that is read like a 125khz RFID when is close to the ignition switch and allows to unlock the injectors if the key/code/data is valid - works as protection - hitag2 is not clonable but have pages of data like for example a Mifare Classic or Coges key.
Now the issues … The radio on the remote to open and close the doors have to be “inserted” on the car central unit for it to operate. When it looses sync but it’s programmed on the car you can turn on ignition and keep pressed a close or open button for some secs for it to re-sync but it needs for the FIXED KEY PORTION OF THE FOB TO BE ALREADY PROGRAMMED on the car !!! And the way it’s programmed is coollll
Because now you have something like a SBB Pro and you want to inser a new remote on the car so how do you do it ? The remote FIXED KEY is encoded on the 1st, 2nd and 3rd page of the HITAG2 so what you insert is the HITAG2 by programming it on the car and NOT the remote !!!
So i buy a new key and cut the blade so it will rotate the cylinder but to unlock the injector i need the HITAG2 to be programmed and when i do programm the HITAG2 using the programmer (inser it on the car as a valid key) i’m also programming the serial of the remote assuming it’s a pre-encoded HITAG for that “blank” key.
To re-use a cytroen C3 remote for example i have to grab the fixed portion of the “key” the remote itself sends and then encode that on 1st 2nd a 3rd page of the HITAG and then insert the HITAG2 on the car by OOB programmer…
You can’t simply make the car accept a flipper made remote or whatever remote by programming that on a car like on a garage door … you have to know how to extract the fixed portion of the remore code and encode that on the HITAG because what you insert/programm on the car is the HITAG and with that you are programing the remote as well.