micro_ros_arduino VS ros2_serial_example

As far as micro controllers go, it's possible to implement a ROS node on both a raspi pico and arduino that allows them to share data with higher level nodes. That said, these specific implementations are a feature of ROS2. Previously, under the the original ROS, microcontrollers were more treated as 2nd class citizens and relied on a rosserial interface node to communicate with the full network.

I recommend using the Arduino library for Micro-ROS. https://github.com/micro-ROS/micro_ros_arduino

MicroROS is a project to implement a complete ROS2 node in your microcontrollers. The experimental MicroROS Arduino branch extends Arduino programs with additional C/C++ headers for full ROS2 compatible message subscription and publication. Specifically, its provides as precompiled binaries that support specific Microcontrollers such as the Arduino Nano RP2040 Connect or the Teensy 4.1. The list of natively supported ROS messages is impressive.

Hint: You may get some ideas on how everything works from https://github.com/micro-ROS/micro_ros_arduino/blob/foxy/extras/library_generation/library_generation.sh

ROS2 does not provide a package similar to rosserial. A technical study how serial communication could be implemented is the ROS2 serial bridge. The basic idea: Incoming data from the ROS2 network is serialized and send to the microcontroller, and received messages are translated to ROS2 messages. However, this not a concrete library.