How it works
Each file is an object with a key (its path), the bytes and some metadata, inside a bucket. Apps usually upload through signed URLs, so the browser sends the file straight to storage, and serve files publicly or through short-lived links. The database keeps only the key.
Amazon S3 set the standard API and most others speak it too: Cloudflare R2, Google Cloud Storage, Backblaze B2, Supabase Storage. Prices are per gigabyte stored plus, on many providers, per gigabyte downloaded (egress), which is where bills tend to grow.