It's a pretty common flow for any higher-power (>100W) power supply. Your PC has one just like that.
The frontend (PFC) produces slightly pulsing DC, say in the range of 350-400V, so it's the first AC->DC stage. It's pulsing because a 1F grid input doesn't deliver the power continuously but rather at 120Hz pulses. There is some filtering after the PFC stage, which makes the output, say, 375Vdc with superimposed 120Hz 25Vac. The next stage is the isolation transformer, where you need high frequency (10s of kHz) for reasonable isolation transformer size. So the inverter produces 20kHz mostly square wave (which is still AC) to pass through the transformer, just to be rectified and filtered again to the final DC output. This second stage is where the regulation happens.
None of that has anything to do with regen. Regen happens directly through the motor 3F inverter, which is pretty much naturally bidirectional.