Thanks @lanefu , I can mount UTM and see the Contents folder. However, the volume drive is mounted in read only mode, so that I can't copy u-boot.bin to the folder.
/Volumes/UTM/UTM.app on /private/var/folders/ns/bkcwhmm106j0_scbdvv_x29c0000gn/T/AppTranslocation/7DABB818-AE48-4EBF-9012-FF45CF3B9602 (nullfs, local, nodev, nosuid, read-only, noowners, quarantine, nobrowse, mounted by User). How can I mount the folder with read-write?