When the universe began to boil

How did the supermassive black holes come into being, which today are gigantic sentinels in the center of many galaxies? Initially, one proceeded from the obvious: The giants grew by accreting other matter or consuming black holes, that is, by merging with them. Step by step, from small to medium to giant. But this concept has a couple of problems. First, we have not yet been able to detect the necessary intermediate stages. They should still exist, but so far we have only found small black hole...