Info to add onto my previous comments/info above, for those who might find this info useful...
As I and others have mentioned the Lotoo S1 Lightning to to USB-C cable works on connecting an Apple Device to both the S1 and P6K.
But I saw elsewhere in Head-Fi someone discussing a new adapter from DD-Hifi TC28i (Picture below) that is a Lightning Male to USB-C Female adapter made from machined Aluminum for the housing.
It is an OTG adapter too! (prevent draining devices by charging the other)
It allows you to use a higher quality USB-C to USB-C or USB-A 3.1 cable and connect your devices to a an Apple Lightning product
YMMV but I've been able to connect an iPad, IPad Air 2, iPhone X, and an iPhone 11 Pro to both the S1 and P6K, to several other Amp/DAC combos that have USB-C or Micro USB without issues.
So far it is THE ONLY connector I have that works across all my AMP/DAC devices, and all my Apple devices, previously one cable would work with one combo, but not another.
This will allow me to consolidate and get rid of a number of cables.
I've seen this little adapter being sold through various stores on the net, so not going to pitch one over the other but rather go get one if you have Apple devices as it will help you consolidate. Haven't had time to do any measurements, except with what is between my ears but the impressions so far is it does not add any unwanted signals/sounds and does not color the sound at all.
Bargain for the $25 I paid for it, given I've had to buy some much more expensive Lightning to USB Cables to date
Just wanted to pass this on to those on this forum!