[Mashimo]

But if the lid is closed, how does the moisture escape? How hot does the air get inside the bag?

[k3oni]

Not that hot, maybe 50-60C, enough to get the moisture out of the filament and into the desiccant while not melting your spool or filament. Was thinking to add a small fan to it one of these days, connected to a battery and on/off switch, but it's working really good as it is.

Just to add to this, i print mostly PETG and PETG-CF, barely any PLA.

[Kirby64]

Moisture gets trapped in desiccant. The hot air drives the moisture out of the filament and into the desiccant.