Anatomy & Morphology
The Yellow Arborians are not a single creature, but a highly sophisticated symbiotic organism. Given how far they are developed they could also be considered to be a synthetic lifeform, yet their biological nature makes this conclusion unintuitive for most.
Genetics and Reproduction
They can reproduce by simply splitting themselves and can merge with others they meet. This is how they exchange all information.
Dietary Needs and Habits
One part of them are billions of microscopic worms living off decomposition, the other is the wandering tree. The tree collects minerals and nutrients and grows some sort of mushroom-like structure around its roots. The worms eat this structure, and loosen up the soil, allowing the tree to gather nutrients more efficiently.
Geographic Origin and Distribution
They live on superhabitable planets with strongly chlorinated environments, yet they can settle any planet with land, some water, and temperatures allowing it to be liquid and will terraform it quickly to fit their needs. Under the yellow-, green-, brownish skies of their chlorine worlds, mountains crumble, due to the weakness of the rocks the chlorine still permitted to exist. Oceans of mild bleach and plants which use high-grade plastics for their structures dominate the landscapes. Their homeworlds allow them to live free of the fear of fire since the chlorine in the atmosphere will suffocate it almost instantly.
The worms by themselves are as dumb as every microbe on earth, but the travel between the feeding roots and the soil creates microscopic networks in which they all commute. Over time the worms developed the ability to mark certain tunnels chemically in order to make their lives more efficient. This system grew in complexity until it was a highly complex neural network. The tunnels are their neurons, the worms their signals. No individual worm is a Yellow Arborian, as no individual cell is you. They don´t think on timescales similar to humanoids, a decade is a normal time to make an important decision for them. Complex plans can take centuries or millennia to devise, but that doesn´t matter since biological death is a foreign concept to them.
Perception and Sensory Capabilities
The trees do more than just provide for the worms. They allow them to live off sunlight, are their building material as they can grow a wide range of high-grade plastics, and most importantly are the data storage of their species. Data is stored as DNA and there is nothing they don´t store. The genetic information of every lifeform they come across, environmental analysis of the lands they migrated through, and whatever they can trade from other Yellow Arborians they meet over the millennia.
Civilization and Culture
They were given many names over the aeons by those they met, and not everyone understood what they were. Those who did not understand had an even smaller chance of survival than those who did.
They are among the oldest races in the universe but, for most of their existence, they were just blind forests who migrated slowly across the surface of their homeworld. But then a massive asteroid impacted their homeworld, almost killing all life. Several million tons of rock were thrown into space. One of them held a root and a batch of worms which survived their journey to space. For weeks they drifted along an eccentric orbit until they crashed back down. They had barely survived this horror but the now regenerating Yellow Arborian was fascinated with what he had learned. The high void was much higher than his kind had ever thought. They had once learned to venture over great oceans to get to continents which had drifted away over the aeons. Where there unknown continents in the void? So began an age of innovation. They used the chemical explosives they had learned to use against competing lifeforms once and indeed they found three barren continents which moved around the homeworlds equator. Settling them was hard, but wrapping them into transparent plastic so they could keep an atmosphere proved to be the solution. They sent out more blind probes into the void, but none returned. So they decided to propel them with rock-fire, a tremendously powerful explosive they used to clear emerging mountain ranges which would hinder migration paths. Colonies riding the rock-fire explosions (they developed Orion Drives) settled the solar system and beyond. Everything that could be terraformed was terraformed into a chlorine world and where terraforming did not make sense they para-terraform as they did with their moons. In the long term, they seek to turn every solar system into a Dyson Swarm. Thousands of plastic wrapped rocks surround their suns, turning their spectra green-brown. Rock-fire and chemical explosives have been replaced by light weavers (lasers and light-sails) and fire-snow (antimatter) long ago, while they spread through a thousand solar systems. Other life has intruded their star forest before, attempting to settle apparently empty worlds. Yet toxic gas attacks, rock-fire, and fire-snow have proven to be extremely effective ways of marking their territory. The ancient asteroid impact showed them the void, but also the dangers of the universe. So They must expand in order to avoid extinction.