A major landmark of Nuka-Town USA, Fizztop Mountain is attached to the remains of the Fizztop Grille, now converted to be the living quarters of the Overboss. Originally intended to be used for maintenance staff, the interior of the artificial mountain now serves as the headquarters for the Disciples, a blood-thirsty raider faction who have added wooden structures, cages, prisoners, and other raider decor.
Immediately from the entrance, sticking to the left will lead to a series of ramps that eventually wind up to the top of the central structure. Taking the right will lead to a hangout area for Discples grunts that circle around to the back and terminates in a dead end. Three makeshift huts exist: one at the bottom encountered immediately upon entering for the grunts, one halfway up the "mountain" where Dixie and Savoy take turns sitting in, and Nisha's personal quarters at the top.
A small cache of chems can be found across the long steel beam that extends from next to the ramp that leads up to Nisha's hut, along with two wooden blocks spelling out "GJ" (presumably shorthand for good job). More than half of these items are clipped into the geometry of the wall and not able to be collected, unless jarred loose by explosives.
The following is based on unverified behind the scenes information and has not been confirmed by canon sources.
Underneath the ramp leading to the cage area where Nisha resides, there is a curious scene of a toy alien using a magnifying glass to view a hollowed out TV set with tiny picket fencing and mannequins inside framed by a tiny rose bush. This is possibly a reference to Romeo and Juliet.
A small image of Han Solofrozen in carbonite can be found on the top of the building inside Fizztop Mountain. It can be reached through creative parkour or the jetpack modification for power armor. It is next to a camp fire on top of Nisha's hut.
End of information based on unverified behind the scenes information.