What are Dyson spheres, and how do we look for them?

Dyson spheres are hypothetical artificial megastructures built around a star to collect all of its radiant energy. https://www.space.com/dyson-sphere.html

https://en.wikipedia.org/wiki/Dyson_sphere

https://theconversation.com/dyson-spheres-astronomers-report-potential-candidates-for-alien-megastructures-heres-what-to-make-of-it-230364

https://www.popularmechanics.com/space/deep-space/a60780331/dyson-sphere-evidence-alien-civilizations/