military flag, standard
旟 centers on a military flag or standard, a banner used to identify and rally troops in battle.
旟 combines the flag radical 方 with 與, which likely contributes the sound and may relate to raising or lifting, fitting the idea of a raised military standard.
The left side 方 is a flag, and the right side 與 looks like hands lifting something up. Picture soldiers hoisting a military flag high on a pole.
For ヨ, imagine a general shouting 'Yo!' to raise the military flag: Yo -> ヨ, and the standard goes up.