Is the Technic hub putting the battery in series or parallel
The batteries are in series. Which gives (in theory) 6x 1.5 = 9V.
Actually it is a little less than 9V as batteries drop voltage a little when they are used.
They can drop as low as 1.2 V when they are nearly empty (giving about 7.2V minimum voltage).
The motors can make do with as little as 7V so they keep working with nearly empty batteries. The hub most likely (I'm not 100% sure) has an electronic cutoff switch that stops it from working at all if the voltage drops too low for reliable operation. This also prevents problems if one battery empties earlier than the others or someone inserted a mix of empty and full batteries.
When using multiple batteries in a device they should ideally be from the same brand, more or less same age and same usage level. Mismatched batteries are never a good idea in multi-battery operations.
Please note: Measuring output voltage on a battery is complicated. If you just apply a Voltmeter to the battery you measure without power being drained (aka without load, the Voltmeter itself is negligible). When under load a battery usually has a lower voltage then when unloaded. How much lower depends on both the load and the condition of the battery.
So it is really problematic to give you a precise answer on &quot;how many Volt does it give&quot;. It all depends.