A player-built map has no cooked level to travel to, and its geometry is far too large to replicate. This is how I got one onto every client in a shipped UE5 project: store each piece as an id and a transform, replicate the save bytes as a property on an always relevant actor rather than an RPC, and let every machine rebuild the level from its own cooked content. Covers the compression header, the size ceiling and what to do when a map is over it, why a pending state is not a failure, the two hashes that stop a client building a level that does not match the host, and the trap that cost me the most: pieces left movable make the client store one as a movement base, which the server cannot resolve, and the player gets pulled backwards. Plain C++ and engine replication, no plugin.
https://dev.epicgames.com/community/learning/tutorials/eDnPq/unreal-engine-sending-a-player-built-map-to-other-players-without-sending-a-file