Dominic Giampaolo of BeOS/BeFS fame. He now works on APFS at Apple. Their work is really - impressive APFS was announced in June 2016 and rolled out on iOS devices in March 2017. Given that the APFS roll-out was relatively uneventful and how they tested it [1], it seems that they can still do low-level engineering and proper testing.

Of course, until recently they had Chris Lattner as well.

[1] For some iOS releases, they converted HFS to APFS in-place, report the results back to Apple, but did not write the APFS 'superblock' to keep the filesystem HFS+. It's quite a smart idea, because they got reports from millions of devices without actually switching them to APFS.