Dropbox

How this question breaks down

1. 1ChunkingCore23 minStore every file as a list of 4 MiB blocks named by their SHA-256 hash, so an edit uploads only the blocks that changed and a block that already exists is never stored or sent twice.
2. 2SyncCore24 minCommit each new file version as a blocklist into a per-folder journal, tell every other device that something changed, and let each device pull the journal entries after its cursor and download only the blocks it lacks.
3. 3ConflictsCore25 minDetect two devices changing the same file by comparing each commit's base revision with the latest one, never lose either side's bytes, and settle what cannot be merged by keeping both as a conflicted copy.

Work through the parts in order and you’ve covered the full design. Each part is one topic.