Are 6in4/SIT tunnels always 1 node to one node, and if that node is a router, does that mean that all the machines connected to that router will be able to use IPv6 internet?
That is mostly the ideal situation. Anchor the tunnel on a router, have the router treat the LAN side as native IPv6. That's why we route a /64 (and optional /48) via the ::2 IP of the tunnel's remote end.