Added1
The FAQ answers how NMTS can run without charging anything.
The answer is what the server does not hold: your files live on the Walrus network as encrypted pieces you bought yourself, and the server keeps only the encrypted list with counts, sizes and times, so it carries none of the disks and traffic a file-holding service pays for. One developer runs one server; trial credits come from a donation pool. The same sentences are in the machine-readable summary.