It's an ecosystem that can use several families of chips, from 8 bit AVRs to 32 bit ARMs. The Arduino code you find online will most likely work on either one, as the most used low level functionality (timers, GPIO, etc.) are abstracted by a high level API that will work on any microcontroller supported by the Arduino IDE.
The RaspbeeryPi ecosystem is too tightly coupled to that very specific Broadcom chip family, which gives them and the RaspberryPI foundation, a powerful monopoly.