Provided are a method by which a first device performs wireless communication, and a device supporting same. The method may comprise the steps of: receiving sidelink control information (SCI) including scheduling information for a physical sidelink shared channel (PSSCH) from a second device through a physical sidelink control channel (PSCCH), wherein the SCI includes information related to frequency resource allocation, information related to time resource allocation, information related to a demodulation reference signal (DMRS) pattern, and information related to a modulation and coding scheme (MCS); receiving first auxiliary information from the second device through the PSSCH; selecting at least one sidelink (SL) resource on the basis of at least one of at least one SCI or the first auxiliary information; performing resource reselection for the at least one SL resource; and transmitting an auxiliary information request to the second device on the basis of the resource reselection.