Great read from Silicon Labs on comparison of isolation technologies namely optocouplers and digital isolators. Link is here
Optocouplers have been the unchallenged signal isolation solution for more than four decades, but digital isolators fabricated in complementary metallic oxide semiconductor (CMOS) process technology are gaining favor in the design community because of their superior performance, reliability and integration.
Fascinating article!
I had a go a while back at decapping a USB isolator to see the little transformer coils used in it –
http://imajeenyus.com/electronics/20131025_ic_decapping/index.shtml