Confused about opamp's inner "ground"
Because the output gain of an opamp is so huge, it doesn't really matter whether the ground symbol for the output voltage source is the -ve rail, the +ve rail, or some point midway between the two, the errors moving it between those points will be dwarfed by the amplifier input offset voltage error.
The ground symbol is probably shown due to an overabundance, but obviously inconsistently carried through, caution on the author's part against showing the output voltage source referenced to nothing.
The output voltage source will source current from the +ve rail and sink current to the -ve rail. The actual point at which the gain equation is referenced to is some point between the rails that's established by ratios of resistors, and either explicit or stray capacitors, so it will vary with frequency.