That would be a question for Amal or someone similarly experienced with encapsulation. There’s roughly a 200C overlap between where glass is malleable (glass-blowing temperature) and where the magnet will deteriorate. Those numbers fluctuate depending on a few factors, including the amount of time the magnet is heated (a magnet can lose it’s juju at as low as 80C if exposed long enough)
They may use a mold, or my guess would be they use like a glass tube, stick in the magnet and some epoxy, and once it’s set, seal the end. Keeping in mind I’ve no idea about manufacturing these things, and the slew of other nuances behind not breaking the magnet.
If you’re Suuuuper eager to get a magnet, Silicone works, but is a bit bulky, and not as sensitive as the TiN coated ones. Massive downside to TiN magnets though, is they’ve got a life of two years or less, before the body breaks the coating and starts to neutralize the magnet/absorb it into the body, as @Satur9 can attest to.
Some of my info may be off though, it’s been a couple years since I’ve done hard research into sensing magnets.