My WIFI is connected to the Internet. I want share Internet with a device that doesn’t have wifi via the WIRED ethernet ports.

Using systemd-networkd I configure /etc/systemd/network/wifi.network like so:

[Match]
Name=wl*
[Network]
DHCP=ipv4
LinkLocalAddressing=no
IPForward=ipv4

Notice IPForward which is key.

My WIRED ethernet port is configured by /etc/systemd/network/wired.network like so:

[Match]
Name=enp0s25
[Network]
Address=0.0.0.0/28
LinkLocalAddressing=no
DHCPServer=yes
IPMasquerade=yes

Notice my DHCPServer so that the client to the network port can get an IP.

I use this networking technique when wire sharking.