Tiny Bridge is unlocked by reaching level 80 using any character on Inverse Gallo Tower (you most likely will already have this unlocked)
Load the level and immediately sprint left, I recommend using Queen Sigma as you won't have to worry about dying and have max move speed. The Chaos Altemanna will show on the Milky Way map but is 5 tiles West of the immediate start location.
Visual Guide (I incorrectly say Tiny bridge is always available, apologies):