After studying statistical mechanics, I understood that thermal fluctuations arise when the system of interest is in contact with a reservoir at some temperature $T$ exchanging energy. Because of this exchange, the energy of the system can fluctuate and its deviation is proportional to $T$.
If this is so, then if a given system is in the microcanonical ensemble, it doesn't have thermal fluctuations (because the microcanonical ensemble has fixed energy and is completely isolated). I find this idea confusing. For example, it is said that magnetization in a magnetic material above the Curie temperature isn't possible because of thermal fluctuations but... that means that if we isolate the system then it can get magnetized at any temperature? This can't be right.