The Raspberry Pi board looks like an Arduino microprocessor but it is more than that. It is basically a full computer that can run its own operating system and can plug directly into a display and be functional.
The main reason to run an Arduino is that it is set up to easily allow you to plug in electronic components like servo motors, sensors to do things like build robots.
The Raspberry Pi is more of a computer programming platform which often runs a linux operating system like Unbuntu or Raspbian and allows you to program in languages like Python, Java, Scratch, Node Red or others.
Python is the most used language on a Raspberry Pi.
It is about 40 times faster than an Arduino in processor power.
It can’t run Windows and needs a graphics processor and storage like an SD card so it is not really designed to be used as a computer workstation other than for programming.
It sells for a little more than an Arduino (about $80) and has less GPIO ports. There are 3 sizes of Raspberry Pi boards
There are many versions like Raspberry Pi 2, 3 4, 4B etc
One of the competitors is Banana Pi.
(under construction Oct 11, 2021)