AFR per bank or per cyl?

With O2 sensors located often one per bank, I'd always assumed that the ECU was only aware of the AFR on a per bank level....however is this how it works or do ECUs know the AFR based on which cylinder was just fired / exhaust stroke?

Per bank only, there is too much reversion/pulsation/mixing occurring once the gases are past the collector to read each cylinder individually.

Thank you. I thought it would be a bit of a stretch to read per CYL.