The opening scene, a dream sequence prior to the vaudeville routines which follow, is what makes this film famous. In it Keaton plays everyone in a theatre simultaneously (through multiple exposures). He is the band leader, all its members, the dancers on the stage and everyone in the audience.
You can stream The Playhouse (1921) free and legally on OldReel, via the Internet Archive.
Yes — it is in the public domain and can be watched and shared freely.