A pi, much more like a computer, has a bootloader program or firmware which has as one of its tasks to load an operating system from a hard drive.
Yes, the Raspberry Pi is a complete computer. I'm not sure if an operating system is required but it can run one, and it has ports for a mouse, keyboard, and display.
The Arduino is more low-level. It doesn't have an operating system and it's more for embedded systems where you need a microcontroller to do something specific without a traditional keyboard, display, or hard drive.
At Arduino takes much lower level commands from us to control ins and outs whereas a pi/computer takes much higher level commands. But basically the atmel328 chip is the equivalent of the intel x86 chip
And whatever code we write in the Arduino ide gets translated to whatever the atmel chip speaks much like a c program gets translated into whatever the Intel chip speaks??
At the processor level, they are both running machine language. C/C++ programs (or another high level language) is compiled to machine language. You can use C/C++ to make a program for either platform, but there are machine-specific C++ commands (especially for the Arduino) and each uses a different complier to create the specific machine language binary for the particular machine.