Refillables are actually not very common among designer scents since they don't seem to care that much about 'sticking to one fragrance'. I think they rather see you just buying a new bottle or even better, their latest release.
However, with some niche brands such as Annick Goutal, refills are usually larger bottles (around 125 ml or even way more), that are shaped different than the 'original' bottle, and usually have no spray attached. They're just splash bottles that are supposed to be decanted into the spray bottle.
Basically the juice in these refill bottles is better value, but they're not always available for every fragrance.