Does a folder have to be stored twice first?

With the native folder stream the sender builds no complete archive and no complete encrypted send cache. It examines and hashes the original files and reads their blocks for the transfer. The sources must stay unchanged until the final tree receipt. If a source file changes, a new job with new keys is required.

On B the data is cached safely and verified at the end. With updates, quarantine or prepared folder states, old and new content can need space at the same time. So the function does not promise “zero extra storage”. The browser route, by contrast, explicitly prepares a local archive copy, so browser space must be free.

What a folder stream carries over

Regular files, directory structure, empty folders and, only with explicit release, the executable flag. No symlinks, device files, owner or ACL transfer, setuid/setgid or automatic program execution. After the transfer pipeit starts neither scripts nor services by itself.

Limits for folders: at most 8 GiB including stream header and layout, 10,000 files, 10,000 directories, 32 levels, 240 UTF-8 bytes per path and 80 per path segment. A long Unicode character can need several bytes. A limit error asks for a suitable split or a different supported job; it never silently leaves files out.