Wireguard might be what you want. You connect to your remote machine ( assume it is at home). You can setup what traffic goes over wireguard (some or all). On your home machine you can run port forward command and masquerading command once connected on home machine so that you have full lan access too. It is described in the wireguard setup docs.

I’m not sure if you can do it without authenticating on the remote. Have you seen sshuttle? Maybe you can run that on the remote to connect to the local machine. If the issue is that the remote “can’t see” the local machine to ssh into it then you could try something like reverse tunnel the ssh port to the remote, and then use sshuttle to connect to the local port that is forwarding traffic.

You can but it kind of sucks and you would only normally do it on a very temporary hacky basis. Otherwise use OpenVPN or Wireguard or whatever. If you want to do it with ssh, see the VPN instructions on the man page. But I mean it’s janky.

Sshutle? It essentially makes traffic go through ssh from anywhere to anywhere. If you ssh into your machine, use sshuttle towards your local machine, I think you should be good

Try googling something like “how to make a VPN server”

Wut. C’mon.

If I’m reading this right, you just need to learn about routing. SSH has nothing to do with this. This is basic networking at best.