The short answer is that we don’t know how supermassive black holes form. However, we do know that once a black hole is already formed in the center of a galaxy, it can grow pretty fast by accretion and/or by merging with other black holes when galaxies collide.
There are currently a number of scenarios that are under study for the formation mechanisms of SMBHs (supermassive black holes). One of them is that the massive stars, early on in the formation of a galaxy, produced black holes tens to hundreds of solar masses. These original black holes then grew by accretion.
In another model, a large gas cloud forms a supermassive star and it collapses into a black hole of about tens of Msun, then it grows by accretion. A third model predicts that the seeds of SMBHs can form when a dense star cluster undergoes core-collapse and matter coalesces into a massive black hole. The fourth model predicts that “primordial” black holes formed moments after the Big Bang.
Here is a representation of these formation mechanisms from your textbook (Begelman & Rees), p100.
The bottom line is that we do not know what starts the process of formation of SMBH, but we do understand how they can grow once the black hole seeds are in place.