There are trim-equivalent commands in the ATA, SCSI, and NVMe command sets. So the OS can issue the SCSI command to the USB device that's using UASP, and the bridge chip inside the external SSD can translate that to the ATA or NVMe counterpart before passing it on to the SSD controller behind the bridge. Not all external SSDs or bridge chips actually support trim passthrough, but these days it's standard functionality.
Apparently macOS doesn't expose the ability for userspace to manually issue trim commands to block devices (or at least doesn't ship a suitable tool to do so), so the best available workaround is to tell the filesystem layer that it should do automatic trimming even on external drives.
