A script to configure libcomposite USB Ethernet Gadget that will be recognised by Windows based on https://github.com/RoganDawes/P4wnP1/blob/62b5c40d9461e4b80998c54830625d6c855833f0/boot/init_usb.sh

usb-rndis.sh

#!/bin/bash
cd /sys/kernel/config/usb_gadget/
mkdir -p pi4
cd pi4

echo 0x1d6b > idVendor # Linux Foundation
echo 0x0100 > idProduct # change to get Windows to rescan while testing
echo 0x0100 > bcdDevice # v1.0.0
echo 0x0200 > bcdUSB # USB2

echo 0xEF > bDeviceClass
echo 0x02 > bDeviceSubClass
echo 0x01 > bDeviceProtocol

mkdir -p strings/0x409
echo "fedcba9876543212" > strings/0x409/serialnumber
echo "Ben Hardill" > strings/0x409/manufacturer
echo "PI4 USB Device" > strings/0x409/product

mkdir -p configs/c.1/strings/0x409
echo "Config 1: RNDIS network" > configs/c.1/strings/0x409/configuration
echo 250 > configs/c.1/MaxPower
echo 0x80 > configs/c.1/bmAttributes

# Add functions here
# see gadget configurations below
# End functions
HOST="00:dc:c8:f7:75:14"
SELF="00:dd:dc:eb:6d:a1"
mkdir -p functions/rndis.usb0
echo $HOST > functions/rndis.usb0/host_addr
echo $SELF > functions/rndis.usb0/dev_addr

mkdir -p os_desc
echo 1 > os_desc/use
echo 0xbc > os_desc/b_vendor_code
echo MSTF100 > os_desc/qw_sign

mkdir -p functions/rndis.usb0/os_desc/interface.rndis
echo RNDIS > functions/rndis.usb0/os_desc/interface.rndis/compatible_id
echo 5162001 > functions/rndis.usb0/os_desc/interface.rndis/sub_compatible_id

ln -s functions/rndis.usb0 configs/c.1/
UCD_DRIVER=$(ls /sys/class/udc | cut -f1 | head -n 1)
echo $UCD_DRIVER > UDC

sleep 0.2
udevadm settle -t 5 || :
#ifup usb0
sleep 2
service dnsmasq restart

While this matches the P4wnP1 script it doesn't work on my Windows 10 machine, but it's the starting point. It works with Linux and I'll test it on a Mac later.

Yeah, this doesn't seem to work on OSx either.

any breakthrough yet?
I've tried different script but since they are all for the pi 0, I don't know if it is really compatible for the pi4 which I try to use with. None of them works. Only the one of geekman and yours are giving me a RNDIS device at least but the drivers can't be found apparently.
That's the result you had too ?

seems that https://gist.github.com/ianfinch/08288379b3575f360b64dee62a9f453f found the solution