Clients often use a "rarest-first" strategy, prioritizing pieces that are least available in the "swarm" (the group of peers sharing the file) to ensure overall file health.
rarest-first piece selection method - Feature Requests - Forums Fragmented Torrent Download
The process relies on a distributed network rather than a central server: Clients often use a "rarest-first" strategy
A large file is divided into numerous small segments called pieces . Fragmented Torrent Download