Which type of seismic sensors usually used in Earthquake Early Warning?
Accelerometers, preferably triaxial (i.e., those that record horizontal and vertical motion). Beyond that your question is indeed a bit too open-ended. Some use many cheap MEMS (http://onlinelibrary.wiley.com/doi/10.1029/2008GL036572/epdf; https://www.theverge.com/circuitbreaker/2017/7/13/15963996/raspberry-pi-seismograph-early-warning-earthquakes; these are similar to those found in phones), some use more expensive but reliable equipment (https://kinemetrics.com/post_products/episensor-es-t/)...