You need to connect something with an ethernet cable to the internet, but there is only Wifi and all you have is a Raspberry PI?

No problem, all you need to do is connecting it to the Wifi, plug the ethernet cable in and tell it to forward all traffic from the one interface to the other, as described in the Arch Linux Wiki.

sysctl net.ipv4.ip_forward=1
iptables -t nat -A POSTROUTING -o wlan0 -j MASQUERADE
iptables -A FORWARD -i wlan0 -o eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -i eth0 -o wlan0 -j ACCEPT

Since our Raspberry is now a router, it should also assign IP addresses to the devices connected to it via DHCP, for example with dnsmasq and the following configuration in /etc/dnsmasq.conf:

#disable dns


This is also a good opportunity to route all traffic through a VPN, by replacing the wlan0 interface above by the configured VPN interface (e.g. tun0 for OpenVPN or wg0 for WireGuard).