An Arduino is a development board, not really intended for an OS (but people have tried it). It is used to control and sense via GPIO’s, general purpose inputs and outputs that can be set and read in the Arduino code you write. My Mega 2560 has just a USB device port and a supply connector. Basic but used everywhere in development, like in IoT. A Raspberry runs an OS, there are quite a few ported already. It has ethernet, USB host, HDMI out, micro USB, an SD card slot and GPIO’s. It is a complete computer, connect a keyboard mouse and monitor, use an SD card with raspbian or another OS and you boot a computer, not so powerful as the intel and AMD desktops but still. It is used as media center, streaming video, or to surf the web, as VPN server, NAS or to edit documents. They have a different use.
Hardware and software advisor for tech startups. ASIC, FPGA, RPi, Arduino, AI, robots, drones, blockchain, Machine learning, vision processing, IoT and 3D printers are my fields of expertise.