ISP can know what about hotspot connected Apple devices?

All this is a hypothetical situation
Suppose I’m using a iPhone (A) with sim in it and vpn active then I turn on mobile hotspot on A iPhone to connect 2 other iPhones or iPads (B & C)(which are without sim) again both with vpn active.
My question is…
Is there any possibility that ISP could know how many devices are connected to iPhone A and which devices are those, like: iPad or iPhone?
Second question is : suppose ISP can know number of devices connected then can ISP know device B and C Mac number, IMEI number, etc?
Third question is: Suppose ISP knows number of connected devices and their Mac number etc. Then, can ISP know which apps are being used on B/C devices?

  1. Assuming that all tethering traffic goes through the VPN, I would say no, but I don’t know how iOS does that.
  2. I don’t think your ISP is able to know MAC number or IMEI number of the devices that are not directly connected to it, so I think that information should only be known of device A.
  3. ISP shouldn’t be able to figure that out unles the VPN leaks, in which case they can likely say which domains you visit and possibly be able to figure out your display size and similar if your traffic is unencrypted. I understand this to be why Tor Browser wants you to always have a specific screen size.

See also:

CC: @nitrohorse at least is an iOS user and can hopefully provide more accurate answers.