The process relies on a distributed network rather than a central server:

A refers to the BitTorrent protocol’s method of breaking a large file into small, uniform "pieces" (typically 32 KB to 16 MB) that are downloaded non-sequentially from multiple sources. This decentralized approach allows users to download and upload different parts of a file simultaneously, significantly increasing speed and efficiency. How Fragmentation Works

rarest-first piece selection method - Feature Requests - Forums

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.

Pieces are downloaded in a seemingly random order based on which peers have them and which are fastest to provide them.