They do it by building their own super high capacity, super low cost storage pods, deduplicating data and relying on the fact that most people don't have multiple TB of data on their personal computers.
But people also tend to use Linux on machines with lots of disk space, like home servers. Wouldn't it be nice if those could backup their 30TB locally attached storage with the unlimited plan? Nope, not for Backblaze, which is why they shy away from Linux and try to get Linux users to buy their pay-per-Gigabyte plan (for which they miraculously are able to provide Linux support...).
-- Backblaze
In other words - instead of backing up your data - backblaze may delete it unless you read and understand their fine print.
I'd never deal with provider sneaking in such a nonsense behind loud claims of "unlimited anything"