Schematic illustrating the formation channel of FN Sgr. A non-magnetic WD is first formed through CE evolution when the more massive star in the binary fills its Roche lobe as a TP-AGB star. The companion of the WD in the resulting post-CE binary evolves and becomes a sub-giant. Meanwhile, the WD ages and begins to crystallize, which creates the conditions for a weak magnetic field to be generated. The magnetic field initially remains deep inside the core but eventually, after a few hundred million years, diffuses and penetrates the surface, becoming detectable. In the meantime, the sub-giant star evolves, becomes an evolved FGB star, and starts to transfer some mass and angular momentum through stellar winds to the WD. As a result of the angular momentum accretion, the WD spin period decreases to minutes, and its magnetic field reaches the super-equipartition regime and is amplified, according to the rotation- and crystallization-driven dynamo. Subsequently, the binary evolves to a SySt, hosting a magnetic WD that is accreting matter through atmospheric Roche-lobe overflow.