Does antibiotic resistant happen because of people not completing their dosage? or, is the antibiotic not strong enough, and therefore the bacteria becomes resistant?

All of the above and more. Majority of infections are treated empirically without a clear strain isolated and tested against antibiotics in the lab. All bacteria have mechanisms to evolve and evade immunity or antibiotics. From there on it becomes a tight game.

All above can induce resistance including over treatment with antibiotics, not finished course of proposed treatment, longer than recommended duration of antibiotics. Also not deescalating antibiotics if feasible can contribute to inducing drug resistance.