Internal voltage is chemistry dependent. In other words, cell manufacturer invariant. It's 3.7V for NMC, 3.2 for LFP, 2.3 for LTO, 3.0 for Na-ion.

Technically speaking, the pack voltage as well as Ah rating should be that of the pack and not cumulative total of the pack; two NMC 18650 in series should be 7.4V 2600mAh, not 3.7V 5200mAh. But denoting as if all cells are in parallel allow this figure to be maximally inflated and so that's what manufacturers do.

High voltage charging etc are not relevant. Though, high voltage assembled battery packs should be marked in that high voltage amp-hour ratings.

The technical reason why amp-hour rating exist is because there are parameters dependent on amperage than energy or voltage, such as thickness of the wire to be used in the device or cycle life of the cell. Voltage of a battery also kind of change proportionate to remaining energy in it, and values like 3.7 for NMC or 1.5 for Alkaline is a 50%, averaged, state.