At my university, we carry around sheet music in black folders in backpacks. I imagine this practice is not the same among professional musicians, such as those in an orchestra or a big band. How do professional musicians normally carry around music?
In the big band, a flight case holds all of the orchs, which get set up by the band leader or one of his minions, on the music stands on the bandstand/stage. That way, no-one ever forgets their music, or brings the wrong folder, and the dots never get lost.
For personal use, there is a special sort of leather case, which has a metal bar to close it over. Never found why it was designed like this, but musos still occasionally use one. 50 or so yrs ago, most musos would use one.