As far as I understand it, there’s no AF “sensor” on mirrorless cameras apart from the image sensor itself. They just adjust the focus until maximum contrast is achieved - but obviously it’s more complex than that.
Recent sensors have AF phase-detect sensors built into the image sensor, by splitting one "pixel" into two sensors (and having lots of them; sometimes in Canon sensors more than half of the pixels have this functionality; on some cameras you can extract the raw image including the different data on those pixels for bigger resolution). The difference between phase detect and contrast being that a phase-detect sensor will give exact values on how to adjust the focus, whilst contrast detection needs focus hunting in small adjustments, so it's slower.