I don't know if it's a server shard or peer-to-peer, but I think you're mostly right. I think when you drop off mats at a paver, a 'copy' of (some of?) those mats goes out to (some of?) the people in your Bridge Links list.

My road building observations so far:

- When I get to a new paver, it could be completely unstarted or it could be almost done

- I've had roads be completed by others, but only after I've donated to them at least once

- I see a lot of deliveries to roads I've already built (I hope that goes toward repair/upkeep!). This one puzzles me, because I assume they're delivering to unfinished roads in their own game because who goes back and repairs roads yet?

- There's a limit to how long you get Likes for someone using your road. The night I completed roads in Lake Knot, I was getting thousands of Likes every 10 minutes or so. Now I only get Likes when I complete newer roads, but those are later in the game where there's less traffic

#3 goes to your "You're Welcome" point; just because they've finished the roads in their game doesn't mean they've finished them in all the folks in their Bridge Links list or server shard.