If you are asking how the fuse (actually a switch) works, it is a simple bimetal disk formed into an appropriate dome shape. Contact is from the top of the dome and the rim. When the higher expansion coefficient metal on the outside of the dome expands enough it forces the dome to invert breaking contact.
Depending on the metals and exact shape of the disk the inversion can be permanent, or reverse when the temperature drops again.
In most cases the switching temperature is set at the manufacturer, but there are rare cases where an adjustment screw allows some tweeking.
Googling "bimetal thermal switches" and "snap disks" should get you enough reading material to keep you going for weeks.