Many handheld multimeters cannot measure more than 100 megohms. Usually, it reaches 600 megohms. Megohmmeter insulation resistance meter can measure the range of megohm or hundreds of megohms; some models reach megohms!
The reason why the multimeter cannot measure such a high resistance is because the measurement is performed at a low voltage (usually a 9V battery is used inside the multimeter). The multimeter circuit provides a constant current, which will measure the voltage across the probe to calculate the resistance. If you use a 9V battery to drive the current source, you need to measure a very small voltage difference to have a high resistance measurement capability, and the resolution of the multimeter ADC is limited. In addition, for wiring and systems that use operating voltages higher than 9V, it is important to test at these higher voltages because the insulation resistance will change at higher voltages. Ordinary multimeters will not "see", only report open.
Therefore, the solution is to use a higher voltage, which is the role of the insulation resistance tester. Internally, it will raise the voltage to hundreds or thousands of volts. This does mean that insulation resistance meters should not be used on electronic circuits. High voltage may cause component failure. The insulation resistance meter is suitable for passive equivalent circuit use cases. This also explains why it is called an insulation resistance meter rather than an ohmmeter. An ohmmeter, such as found in ordinary hand-held multimeters, is unlikely to destroy the electronic circuits it uses without electricity. Due to the high voltage, such devices are only used in the case of non-electrical wiring disconnected from electronic circuits, non-electric motors or transformers.