A Dyson sphere is a hypothetical megastructure that encompasses a star and captures a large percentage of its power output. The concept is a thought experiment that attempts to imagine how a spacefaring civilization would meet its energy requirements once those requirements exceed what can be generated from the home planet's resources alone. Because only a tiny fraction of a star's energy emissions reaches the surface of any orbiting planet, building structures encircling a star would enable a civilization to harvest far more energy.
That article is not comprehensive either, their universe is quite expansive.
Two of my favorites may be "W-brains," computing structures with very carefully arranged wormhole pairs serving as data buses to overcome the latency of communicating at such scale, and "neural stars," another take which is a computational structure inside a neutron star sized volume/mass (again, to overcome latency issues).
There are much smaller megastructures too, depending on where in the timeline you are looking.