Who composed aleinu?
Yehoshua, upon entering Eretz Yisrael.
Aleinu should be said by an individual in a shul whenever the tzibur is saying it (if it is muttar for him to be mafsik). A person learning Torah in the Shul should stop and say aleinu with the tzibbur.
HaShem will do battle for you, and you keep quiet (Shemos 14:14).
And some people just kept talking,oblivious to everyone’s stares……….