Voltage is always a relative measure, never absolute. That is if you only consider a singular point there is no concept of Voltage.
In electronics, there is almost always an implicit reference which is generally referred to as 'ground' and, as the reference, is arbitrarily chosen to be 0 Volts. When someone talks about a voltage at a single point, it is generally assumed to be referenced to the circuit ground (whatever that may be).
Ground may be a "Real Ground" that is tied to the Earth, hence the name or it may be a "Virtual Ground" meaning that it is only the reference locally to the circuit.
An example would be a battery powered device. Generally the negative terminal of the battery is chosen as ground, 0 volts, and used as the reference for all other voltages in the circuit. However, if you were to measure the voltage between the negative terminal of the battery (a virtual ground) and a true Earth ground, you'll probably find it isn't zero.
V = IR is a formulation of Ohm's Law doesn't doesn't derive directly from the concept of voltage although voltage is certainly a part of it, along with the concepts of resistivity and current.
Ohm's law wasn't actually based on mathematical analysis, it was found experimentally (which is generally true of all 'laws' in science). Its actually only applicable in a fairly narrow set of situations. Specifically 'ohmic conductors' sometimes called 'linear devices' in electronics, things like resistors. 'Non-ohmic conductors' sometimes called 'non-linear devices' do not obey Ohm's law, things like transistor and diodes.
Technically Ohm's law never really applies outside idealized components, that is no real component actually obeys Ohm's law exactly, it is however a reasonable estimate in many applications.
If you want a more technical derivation you can get Ohm's law to drop out of the Drude model pretty easily and they made us do this in one of my college classes.