Asklemmy

There's a random planet in Mass Effect that is tidally locked, and mentions it's life exists only in the twilight region. The light side is violently hot, and the dark side is brutally cold. Every time I come across this planet my imagination always goes a little wild, picturing strange alien creatures lumbering about in the cool air beneath a deep purple sky and perpetual sunset. It's such a cool vibe.

I think you'd like The Sunlit Man by Brandon Sanderson! Came out a month or so ago, definitely worth the read. The setting is a planet where the sun/sunlight kills, literally hot enough to melt the rock of the planet as it goes. Very good book imo

I'm imagining so many diverse civilisations... Some living in the poles, with rotating walls and mirrors to control the heat. Some civilisations living underground. Some are nomads, moving constantly (would work really well for small planets with a very long day). Some civilisations live near volcanos and use geothermal heating to survive the long nights, and hibernating in deep caves in the day.

Later when nuclear energy and heatpumps are invented/discovered, more of the planet becomes inhabitable, with extremely fast underground trains to facilitate travel.

This is the most recent world in Thresholder, if you'd like to read an excellent fic that fleshes out that idea well