There wasn't a thread comparing Arduino to Raspberry on the forum, which is somewhat weird, as both are open source and both have lots of GPIO that intend to power some external hardware.
Before I enter the rabbit hole I had viewed the two to be indifferent. One was in python and one was in c++. One had some USB and one don't.
The difference now become much more apparent.
Raspberry Pi is a lot more powerful than ordinary Arduinos (maybe beside portenha H7) and also is a lot more complicated. It have a lot of RAM, USB slots, HDMI, and it boots, off a micro SD card.
It also doesn't have some of the GPIO on the Arduino, like it lack analog pins.
It's more like a mini computer, with a bunch of GPIO (I2C and SPI too) that allow you to connect some stuff to it.
FPGA is .. well, it's more like ... well, it's a array of logic gates. Really large array of logic gates. Basically you implement your own circuit. You implement your own full adder, arithmatic logic unit, and you implement your own registers and stuff. It's a lot more hardware and fixed-purpose, and because of so it can perform tasks extremely rapidly.
Arduino on the other hand is basically a tool that allow you to program a selection of chips. ATmega32U4, ARM3X8E and the like. These chips are microprocessors, small processors with RAM and flash (maybe EEPROM) built in that can perform pre-programmed tasks. More powerful chips exist, such as the 3X8E, with advanced protocols such as I2S, CAN and capabilities such as "true analog". They offer a good development platform as they are many types with specializations (such as display handling) and often without extraneous features, which keep cost down.
Alongside FPGA, which can be commonly found anywhere from SSD controllers to USB hubs, these chips can be used to make actual products. Bluetooth headphones, for example, use a commonly available, programmable chip from CSR to handle bluetooth. smart response XE for example used some ATmega chip, not unlike the 32U4, for process. The only reason these chips are not identified in Arduino IDE is because they are not using the Arduino "firmware" -- their code is burned into the chip using the ICSP header.