The term "Arduino" is for the ecosystem; it doesn't refer to any singular microcontroller.
At one time, long ago, it did - mainly the ATMega8 platform that was at the heart of the Wiring project, which ultimately begat the original Arduino (and Processing).
But the ecosystem expanded; first via different Atmel microcontrollers (many which became so-called "official" platforms), then eventually to other microcontroller platforms (and Microchip, maker of the venerable PIC microcontroller platform, eventually purchased Atmel).
The ESP8266 is a part of that ecosystem today, along with other so-called "ESP" controllers:
https://github.com/esp8266/Arduino
https://www.esp8266.com/wiki/doku.php?id=start-with-esp-12-a...
https://github.com/espressif/arduino-esp32
It wouldn't surprise me to find out there is a core available for the Raspberry Pi - or at least for the SOC on the board...
Hmm:
https://github.com/me-no-dev/RasPiArduino
I looked to see if I could find any kind of "real" core for the Raspberry Pi or Broadcom SOCs but I didn't find anything other than the above, which seems to be in active development. Honestly, though, you wouldn't really use a Raspberry Pi that way; more likely you would use the Raspberry Pi as a development platform for an Arduino ecosystem platform, that would likely be attached to the Pi via USB or some other method.
People, especially beginners, are often confused by what the name "Arduino" means; as I've noted, today it's an ecosystem composed of many different platforms, some "official", but many more that are not.
...and this of course doesn't get into the whole "real" vs "knockoff" vs "counterfeit" debate (which is a whole 'nother story).