Why did Allah legislate marriage for humans?

What is the ruling on a marriage contract without announcement and without being documented in court?

Is it valid for the contracting party to be one of the witnesses?

Is it permissible to marry without documentation or witnesses, with a proposal from the man and acceptance from the woman? Is it permissible for her to marry herself if she resorts to such a marriage out of fear that they will reject her marriage, and fear that they will take her children, and she is afraid for herself of falling into sin?

Is it permissible for a full brother to witness his sister's marriage contract?

A girl who is twenty-nine years old said to a young man who is twenty-one years old: "I marry myself to you," in the presence of six girls as witnesses. Is this marriage valid?

A woman asks that a person proposed to her, and they got married, but the contract was not written in court, noting that the contract was made in the presence of witnesses and guardians. What is the ruling on the contract?

What is the ruling on a woman's role in signing the marriage contract?

A man and a woman were with a group of young men and women, and the man said to her: I marry you, and the woman accepted in their presence, and there were witnesses to this. What is the ruling on this marriage?