Hi there,
It’s on-chip in C6.
IMO, unless one needs a specific interface or RF stack or RF performance the S3 is MORE in terms of RAM and Flash and CPU speed, Sleep current is negligible. to me the C6 is Meh’ if you need it kind of MCU. Things like CAN buss and the ZIGBEE, Matter clans will flock to it. but you could also do those with ESP32c3 if you have a shoe-horn. so NOT allot to see here new anyway. it’s an Incremental Espressif progression.
YMMV
HTH
GL 
PJ 