BJT transistor working and construction
First, in normal operation, the base-collector junction is reverse-biased.  This causes free carriers (electrons or holes, depending on NPN or PNP respectively) to migrate across the junction, and creates a depletion region around the junction.

Second, when current carriers are injected from the emitter into the base (by adjusting the base-emitter voltage), those carriers, and the resulting current, have to go somewhere.  Some of them, usually around 1-2%, will recombine in the base, giving rise to base current.  Most of them will feel the attractive force from the reverse-biased base-collector junction, and flow to the collector, giving rise to collector current.

The recombination in base vs. continue to collector ratio is of course the ratio of collector current to base current, and is a design parameter of the transistor.

The above is SIMPLIFIED, with lots of handwaving.