Why do we say Amen?

A newcomer is present in a Christian meeting. He was puzzled when people said Amen together after prayers. He also saw some individuals saying Amen while the pastor was preaching. What is the meaning of the word Amen? The friends who invited him could not give an answer and the meaning of this word. It simply means so be it or certainly. The word is used in many situations in the Bible.
Affirmation of truth: Amen is used to affirm truth in the Old and New Testaments. It was first used in the bible when the curse was pronounced on a woman in the Law of Jealousy. (Numbers 5:22) When curses were pronounced for violation of the Law, people responded with Amen. (Deuteronomy 27:15-26) People of God acknowledged that God is Holy and His judgments are righteous. It was an expression of faith in the truth and attributes of God.
Praises: The children of Israel added Amen when praises were sung unto the Lord. (I Chronicles 16:36; Nehemiah 8:6) Seven Old Testament references to Amen are connected with praises and thanksgiving to God.
Second coming: In the New Testament Amen is used by Apostle John, he ends the gospel and his letters and book of Revelation with Amen. The nine times it is used in Revelation, which is connected to the Second Coming of the Lord Jesus Christ.
God of Amen: Prophet Isaiah called God trustworthy and faithful as ‘the God of Amen.’ The Lord Jesus Christ is referred to as Amen. (Isaiah 65:16; Revelation 3:14)
Promises of God: Paul writes that the promises of God are Yes and Amen in Christ. (II Corinthians 1:20)
Prayers: Generally, Christians end their prayer and petitions with an Amen. It is an expression of the desire of the heart that God graciously answers the prayers.
A Warning and a Mock: Prophet Jeremiah mocked the false prophet Hananiah for his false prophecy by saying Amen. (Jeremiah 28:6) While Jeremiah was pronouncing judgment on the Nation, he prophesized peace. The intention of Jeremiah was ‘I hope so.’
Do I understand the meaning and say Amen?