Supermassive black holes at the center of galaxies
This is a very well posed question, thanks!
The problem is, we still don't know.
What we suppose, is that dark matter came first.

Primordial fluctuations of dark matter made the right conditions to accrete enough mass to build SMBHs, and then the surrounding galaxy.

But still we don't know if, the SMBH comes from merging, of other SMBHs or galaxies, or from high massive BH that are called seed (of the order of $100\,M_{Sun}$).
Perhaps, there are very recent developments, which could be known by the experts, but until few years ago, this was the scenario.

About the mass issue, we could have a better reasoning if we think to the influence spheres. We discovered a relation between the SMBH mass and many bulge properties (i.e., the luminosity, the velocity dispersion, and others). This means that an object of the order of $\sim1$ pc, influences an object of the order of $\sim1$ kpc (maybe $\sim3$). Plus, the bulge structure is essential to the formation and characterization and classification of the host galaxy.