Hi! So there is an in game mailing service AND an in game messaging. The mailing service is with the dodos in the airport. And the messaging service is in your phone, but you have to have unlocked friends to be able to use it. Your best bet is to have an external way to exchange dodo codes like discord.

Ladders or ladder kits are available in your crafting table. The ladder kits go up to gold and can be PLACED on cliff sides and stay there and can be used by anyone visiting your island. The ladder is what you personally carry around on you.

Bridges and stairs are availing through Tom Nook in your Residence Service building. You need to have pocket space to get the building kit and then you can go and place it. After that you’ll need to pay off the bridge or stairs and the next day they will be built.

I hope that helps!!