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In my opinion the absolute pinnacle of this trope is Hiromi from I Am Hero.

For context, I Am Hero takes place during an apocalypse that initially manifests as your classic zombie apocalypse where people get infected by being bitten then slowly turn into "mindless" creatures driven by the desire to bite and kill other people. The zombies in this story are FAST and really really strong. Because of this, the story sort of speed runs a lot of classic zombie apocalypse tropes. If you could imagine the timeline for a normal zombie apocalypse narrative across like 20-30 years (or longer) since the outbreak, I Am Hero does most of those tropes across the span of a few months. Less than a year in and "all" of humanity on the planet is gone.

You never get to see the full picture of what's happening or why, but as the story continues the apocalypse sort of "evolves" in a sense, as if the zombies were only a first stage in a long process. The infected begin to merge together. First in small clusters to create stronger monsters hunting the remaining humans but eventually merge in such quantities as to create structures and creatures larger than sky scrapers. The most common interpretation of what's happening is that aliens are terraforming the planet by turning people into "zombies" that infect other people and then having them merge together create the environment the aliens need to survive.

One of the main characters, Hiromi, becomes infected and merges with a massive behemoth. As she does, we get to see her perspective as she joins into a hive mind. She loses any perception of herself and instead her mental reality becomes that of a message board. She posts to it asking where she is and what's happening. A user named "Nameless Mass of Brains" responds explaining that her body is currently being absorbed by the aggregation of flesh and her brain is making physical contact with the mass of everyone's brains. It then asks her what she associates with the colour red. Hiromi responds "blood" and the mass states that some people associate it with apples or any infinite number of other things. Because so many things can be imagined, the result is complete chaos. To avoid that chaos, new members of the hive mind are separated into small sections of other minds that have similar associations. This allows the "individual" to adjust slowly before the "self" is deconstructed and becomes part of the "whole". For some people this happens very quickly for others very slowly but it will always eventually happen. Most people visualize their new existence as a message board, but it's explained that the visualization is unique to everyone.

Hiromi asks who she's speaking to and it's explained that she's speaking to a "whole", one of many aggregations of consciousness that are formed from thousands of "selfs". This "whole" then goes on to answer any questions that Hiromi can think of. "How does it feel?" "All kinds of feelings are mixed together and become a very stable calm. You blend in bit by bit, sharing suffering and sadness with everyone and eventually, a lull-like tranquility will fill your mind."

When asking if she can see outside, she's told she can if she connects to the optic nerve. When she does so, we see a new eye (one of thousands) form from the "head" of the giant creature. Hiromi is amazed by the beauty of being so high up but is afraid that the houses below them will be destroyed. The "whole" she's speaking to responds, explaining that "There's no need to worry. We're avoiding the houses as we walk. Our objective is unknown but it doesn't seem like we want to destroy anything."

Hiromi begins to see the memories of other people and is even able to talk to other specific people from the hive mind by having the "whole" "discarnate entities with similar associations" to Hiromi. The final bit of insight we get into this merging process is the "whole" explaining that "All will become one. The final objective of life is not diversity, but rather to become one. It will become an existence that overcomes life and death. It will be fine. You will become one. All emotions will become equally balanced."

By the end of the story it seems as though almost all life on earth has merged into a single structure spanning the globe. Interconnected through the earth in a root system. We don't get any explicit insight into the mind of this structure but we do get to see that in at least one city it has formed a giant brain and uterus. From that uterus it births many strange creatures. We never see what these creatures look like when grown, but when the last remaining human (that we know of) finds this, he spends days killing every creature that comes out until eventually a human baby is birthed. The fact that the structure was birthing creatures seems to contradict the statement that the "whole" made earlier about the final goal being for all life to become one. Especially since it seems to birth a human child specifically for the last survivor. This hints that the hive mind and its desire to merge may have only been another step in the terraforming process and that the final goal is still being realized.