You can use systemd-networkd to simply configure the wired ethernet interface. Please follow to Use systemd-networkd for general networking. You can just use section ♦ Quick Step there and then come back here. To configure the eth0 interface, create this file:
rpi ~$ cat > /etc/systemd/network/04-wired.network <<EOF
Ethernet uses an analog signal scheme with five different voltage levels, GPIO pins can only detect digital = two different voltage levels
Ethernet uses differential and not single ended signals
GPIOs usually support switching speeds in the range of a few 10 MHz while Ethernet (100M and 1G) operate on 125 MHz.
Even if a GPIO Pin could read the ...
Network interface names can be managed by systemd, which provides you with a way to use names you like via .link files. E.g. you can create a file /etc/systemd/network/10-internet.link:
Using built-in kernel names such as eth0 may lead to trouble, but you can try to reuse them nevertheless.