To put it another way, with regular simple office usage, in terms of power utilization it will behave like a CPU with integrated graphics for an ultra low power ultraportable. However for heavy workloads, it will behave like a high end workstation with high end desktop CPU and high end mobile GPU, but only drawing mid-tier power levels.
With low to moderate usage, this will be silent. With very heavy usage, fan noise will be audible, but it will be nowhere near as bad as the Intel & AMD laptops with discrete graphics.
In this review, he states the M1 Pro MBP takes something like 10 minutes of Cinebench before the fans even become audible:
This reminds me of my Core i5-7600 iMac vs. Core i7-7700K iMac. With video encoding, the i5 would take 9.5 mins before its fans became audible. In contrast, the Core i7 would have the fans at max in 30 seconds, which was really obnoxious.
To put it another way, with regular simple office usage, in terms of power utilization it will behave like a CPU with integrated graphics for an ultra low power ultraportable. However for heavy workloads, it will behave like a high end workstation with high end desktop CPU and high end mobile GPU, but only drawing mid-tier power levels.