Hi! I am not from the electronics background, so this question may seem trivial. I wonder what is the communication protocol for Arduino when it communicates through USB to laptop. Is it a USB 2.0?
It's more trival than you think. Arduino simply sends a serial signal
Serial.print(ladeda);
to its serial port, and neither knows nor cares what sort of USB it is - 1,2, or 3.
Indeed, it neither knows nor cares what is connected to it - if anything.
On boards like the Uno, Mega, Nano, the primary microcontroller uses serial communication to a USB to TTL serial adapter chip (ATmega16U2, CH340, FT232, etc.) which handles the USB communication with the computer. The data is presented on a "virtual COM port" on the computer, so it looks like you had the primary microcontroller directly connected to an old school serial port on the computer, even though there was the USB part in between.
Other boards like the Leonardo, Micro, MKR have native USB capabilities in their primary microcontroller and so they can be connected directly to the computer, instead of needing the separate USB to TTL serial adapter chip. Those chips still create a virtual COM port on the computer, but there is never that intermediate step where the data goes across the board between two chips as serial communication. It is USB from the very start, even though the Arduino API gives you that standardized Serial.println(), etc. interface. Those boards are also capable of other types of USB communication, such as the HID used to emulate a keyboard, mouse, or joystick.
When you upload a sketch to the Arduino board, that is done over the virtual COM port, but there is an additional protocol layer used for communication between the upload tool and the bootloader on the microcontroller. There are several such protocols, depending on which bootloader is in use.