Once a new building or strategic unit has been put into play, it gives possibility for the colonists to start working there. The colonists are all the time looking for better paid jobs and the employers are looking for the best possible candidates for the jobs. If the offered pay is good enough for the candidate and the skills of the candidate are adequate for the work, the job seeker gets the position. Depending on the labor market situation someone else may or may not get fired and then start looking for a new job.
The labor market in the game is constantly changing and so is the wealth of the colonists. If a colonist family (or a single colonist) succeeds to save enough cash they can then build their own habitation module or afford better education for their offsprings. On the other hand, you can also support your deprived colonists by offering them things like unemployment benefit, free education and free health care from the funds of the colony.