On the GitHub Releases page, files are listed under "Assets." The naming convention differs by operating system.
Head to the LosslessCut GitHub Releases page to ensure you are getting the safe, official version. losslesscut portable download
The official DMG contains a .app bundle. While not "portable" in the purest sense, you can drag that .app to any folder (including an external drive) and run it without installation. No admin rights needed. On the GitHub Releases page, files are listed under "Assets