The annual Day of Atonement for the nation

Leviticus 16:1-22
Aaron is not to come whenever he chooses into the Most Holy Place behind the curtain in front of the atonement cover on the ark, or he will die.
Aaron is to enter the sanctuary area: with a young bull for a sin offering and a ram for a burnt offering.
From the Israelite community he is to take two male goats for a sin offering and a ram for a burnt offering.
Aaron is to offer the bull for his own sin offering to make atonement for himself and his household.
The two goats one for the Lord and the other for the scapegoat- the goat of removal, to be used for making atonement by sending it into the desert as a scapegoat.
Aaron is to take a censer full of burning coals from the altar before the Lord and two handfuls of finely ground fragrant incense and take the behind the curtain. The smoke of the incense will conceal the atonement cover above the Testimony, so that he will not die.
The tenth day of the seventh month was to be the annual Day of Atonement for the nation. Only on this occasion was Aaron allowed into the innermost part of God's Tent.
Why?
Azazel: A place in the wilderness to which the scapegoat was sent, symbolically carrying away the sins of Israel. The meaning is uncertain, but it cannot refer to an offering to a demon, as some suggest, for this was strictly forbidden.
How?
Jesus Christ once be sacrificed on the cross then the atonement is made for ever to all who believes in Jesus Christ and repent to his own sins.