The INS unit is separate and often has its own set of gyros and has to be connected as separate input to FMS or other navigational computers, same as connecting GPSes or other radio nav components.
For example the current model of popular Universal Avionics UNS1 series of NCU (navigational computer part of FMS) come with built-in augmented GPS receiversz but do not mention INS functionality at all even in extended models. Don't have access to manuals at the moment, but I'd expect to see INS as optional to connect over one of the external connectors on the MCU, as it was on the older models without integrated GPS
I had assumed that the ADIRU’s inertial reference data from its gyroscopes and accelerometers would feed into the FMS to provide INS capabilities in case GNSS was unavailable.
ADIRU in its minimal common form (some vendors might make a beefier one, who knows) provides the equivalent of old pneumatic "air data central" + artificial horizon and turn indicator gyros.
GNSS becoming "reasonably easy" to add meant that there was way less push to integrate INS after early use in transoceanic flights
Many small planes don't have INS in typical meaning, but their pilot is INS computer, calculated approximate nav from air data (air speed + weather data + compass or radio compass).
The INS unit is separate and often has its own set of gyros and has to be connected as separate input to FMS or other navigational computers, same as connecting GPSes or other radio nav components.
For example the current model of popular Universal Avionics UNS1 series of NCU (navigational computer part of FMS) come with built-in augmented GPS receiversz but do not mention INS functionality at all even in extended models. Don't have access to manuals at the moment, but I'd expect to see INS as optional to connect over one of the external connectors on the MCU, as it was on the older models without integrated GPS