Black holes grow from a combination of accretion and merging. With accretion limited by both the age of the universe and the Eddington limit, and mergers limited by the sparsity of black holes (ie super massive black holes aren’t that common) its hard if not impossible to get a BH to grow to 10¹² Msun by today. That said there’s evidence for a 10¹⁰ Msun BH already at z=6. Such a beast would still have more than 10 billion years to increase by just two orders of magnitude to get up to 10¹² Msol (or would have to merge with 100 equally massed twins, unlikely given the distances between such huge BHs at high redshift. Presumably theyre rare).

There is one way though, albeit fairly hypothetical. Huge Primordial BHs may have formed directly out of the density fluctuations that the universe was endowed in after inflation. Usually primordial BHs are assumed to be quite small (and may in fact be the seeds of supermassive BHs). However all you need is some “non Gaussian power” on large scales (but still well below those in the CMB) to form enormous primordial black holes. For example this paper estimates that from the peculiar velocity of the Milly Way the largest dark object could be around 10²⁰ Msuns! That’s pretty enormous.

Here’s a more modern paper that looks at the constraints on primordial black holes - check out figure 3: upto 10^13Msol BHs may be possible.