The roots use chemical releasing hem into the soil, which can be sensed by neighboring plant roots. These chemicals can indicate things like nutrient availability, disease presence, or even stress signals. But they also communicate with fungi which helps the roots to absorb water an nutrients. The plants give the fungi other nutrients in return too.

The ultrasonic communication is still under research I think. One theory is that plants produce ultrasonic vibrations through the rapid movement of water within their tissues or the opening and closing of tiny pores. These vibrations could then propagate through the air or soil and be detected by neighboring plants or even insects and animals.

Listen to plants sounds here
https://www.sciencenews.org/article/plant-stress-ultrasonic-click-noise-sound