@nico

> but the whole point is the packets to my ISP aren’t addressed to Google, they’re addressed to my ISP

That's wrong, capture the packets.

If your ISP gives you a default gateway of 1.2.3.4 it does not put the destination in the packet as 1.2.3.4

The destination address will say 8.8.8.8 if you're trying to reach 8.8.8.8.

Your router/firewall says: "8.8.8.8? I don't have that, the most specific route that matches is 0.0.0.0/0 which goes out ens2 to the default gateway 1.2.3.4, so I'll just send this packet there. What's 1.2.3.4's MAC address? Ok, rewrite the frame to use that MAC and send it out ens2 now"

The IP address is not changed.