rosserial VS pymata4

A core package of ROS1 is the Rosserial. It supports several concrete microcontrollers, including the Arduino, STM32, Teensy. Effectively, this package is a set of C/C++ headers that you incorporate in your microcontroller code. Then, you can directly receive and send ROS messages using the imported ROS message format. The receiver, e.g. your single board computer, needs to implement a ROS server. Here, you can use either C++ or Python. There are several examples in the Github repository. I also recommend a great blog article about connecting ROS to Arduino with L298D to directly convert TWIST messages to motor controller PWMs.

3 - On your ros linux PC, you can either download Rosserial (ex. sudo apt install os-melodic-rosserial-arduino), or build rosserial from source (https://github.com/ros-drivers/rosserial). Either way, you'll have a tool that can generate the Arduino side library for you (it will generate headers for all current messages/services... including your custom ones). For example:

The Firmata Protocol provides an abstraction layer for communicating with a microcontroller to read and write its GPIO pins. Currently, it works with Arduino based microcontroller. To use this, you need to install the Firmata firmware on your microcontroller, and then use the client library for sending Firmata commands. The protocol support different client libraries, like Python Pymata4 or other languages like JavaScript and Ruby. There are no ROS abstractions, which means inside the client you would need to write custom code for processing and generating ROS messages.