Once constructed, you decide what to use it for, Residential, Tavern, canteen, sick-bay etc. If you choose Residential, one of your colonist will move in. Buildings have a number of upgrades you can install, the cheapest being a single wooden bed, so they don't have to sleep on the floor.
The colonist's activities through the day is semi-random, They'll wake up somewhere between 3am and 6am, spend the day doing various tasks to keep the colony alive and running. In the evening they'll socialise a lot more, especially if you've built a tavern or other social building. Late evening they may or may not go home, maybe watch TV for a bit and then go to bed around midnight. Timings and what they choose to do is pretty random, but will later be more dependant on their desires (when I've developed that bit!)
Really glad how this is turning out, none of the other "Colony management" games out at the moment work just how I want, so I'm really looking forward to playing this instead of just testing each feature works before moving on to the next thing.