Using Raspberry Pi 4 as travel router to bridge from iPhone USB Tethering eth1 to wlan0 and eth0

I have a spare Raspberry Pi 4 that I plan to build a travel router/hotspot in which it will connect to the internet through iPhone USB Tethering then broadcast to both eth0 and wlan0.
There is an official guide from OpenWRT to enable USB Tethering. But my questions are:
1/ Can also the Pi build-in ethernet eth0 be enabled for this kind of network setup?
2/ Can the whole setup be in bridge mode, which means the Pi and devices connected to it will get the same IP subnet as my phone public IP?
I'm pretty new to OpenWRT but am interested in learning more about this awesome platform as well as exploring cool things/ideas I can do with my Pi.
Thank you in advance,

Yes. By default the built in Ethernet is already in the lan network, so in the standard routed configuration where the phone is wan, the Ethernet port will have Internet access through the phone. Setting up an AP would typically be bridged into lan, so wifi users are in the same local network with the Ethernet devices.

You could bridge eth1 into lan and make the Pi a simple bridge, however there isn't much upside to doing that. The phone will be doing all the NAT routing and firewalling, and in IPv4 it will be issuing IP addresses from a private pool since there is only one public IP.

For IPv6 the phone company gives your phone a /64, which allows tethered lan devices to each have unique public IPs which are the same first 64 bits and different last 64 bits. This would happen inherently in the bridged configuration but is also readily supported in the routed configuration by using relay mode.

Thank you!
I was able to get Internet connection from the ethernet port. I also tried to bridge the network by add eth1 to lan in /etc/config/network. However, I only able to have internet via the wireless but not the LAN. Any suggestion with this? Thank you again