> If they deviate from their flight plan, don't they have to at least inform ATC to basically update the flight plan
When on an IFR flight plan - at least domestically, especially in busy airspaces, the flight plan is mainly sort of a backup in case of lost-comms. You are typically vectored by ATC and your route may or may not be exactly what you filed. For example, in the SoCal area, the "standard" IFR routes can be pretty roundabout going around the approach/departure corridors of the main airports. But, if at any time the area is clear, ATC would typically clear you through a shorter routing.
I am not sure how it works for international routes. I know that for trans-oceanic routing, they typically make position reports over HF radio when over the ocean where there is no ADSB coverage.
When on an IFR flight plan - at least domestically, especially in busy airspaces, the flight plan is mainly sort of a backup in case of lost-comms. You are typically vectored by ATC and your route may or may not be exactly what you filed. For example, in the SoCal area, the "standard" IFR routes can be pretty roundabout going around the approach/departure corridors of the main airports. But, if at any time the area is clear, ATC would typically clear you through a shorter routing.
I am not sure how it works for international routes. I know that for trans-oceanic routing, they typically make position reports over HF radio when over the ocean where there is no ADSB coverage.