Sound Editor: A sound editor is a creative professional responsible for selecting and assembling sound recordings in preparation for the final sound mixing or mastering of a television program, motion picture, video game, or any production involving recorded or synthetic sound. (https://en.wikipedia.org/wiki/Sound_editor_(filmmaking))

Sound Mixer: They visit locations to check for potential sound problems, like passing trains or road noise. During filming, sound mixers ensure audio from radio and boom microphones is recorded at a good level for every take. (https://www.screenskills.com/job-profiles/browse/film-and-tv-drama/technical/sound-mixer/)

A sound mixer works more in production and a sound editor works more in post-production.