As already mentioned the Hebrew word for angel is "malak," and it means "angel/messenger/sent one." How the word is used depends on the context. Someone ask where the word is used in reference to humans.
A good example is at Malachi 3:1. "Behold I am going to send my "malak/angel/messenger," and he will clear the way before Me. And the Lord whom you seek, will suddenly come to His temple; and the "malak/angel/messenger" of the covenant, in whom you delight, behold He is coming, says the Lord of hosts."
The person/messenger that will clear the way of the Lord is John the Baptist. This is confirmed at Mark 1:1-4. So who is the "messenger" of the covenant in whom we will delight that is coming to HIS temple? I'll get to that a little later. As a side note the prophet Malachi who is human, well his name is from the word "malak."
Now, getting to your example of Genesis 19:1. The best way to explain it to start at Genesis 18:1 through the end of the chapter which you can read on your own. I will highlight the main points.
At Genesis 18:1, "Now the Lord appeared to him/Abraham by the oaks of Mamre, while he was sitting at the tent door in the heat of the day." Vs2, Abraham looks up and three men were standing opposite him and he bowed himself.
Vs3, and said, My lord if now I have found favor in your sight, please do not pass your servant by. Vs4, Please let a little water be brought and wash your feet and rest yourselves and Abraham offered them bread etc. They ask where is your wife Sarah?
Notice vs10 where one person is speaking and he says, "I will surely return to you at this time next year; and behold, Sarah your wife shall have a son." Sarah laughed to herself. At vs13, "And the Lord said to Abraham, Why did Sarah laugh, saying Shall I indeed bear a child, when I am so old." Vs14, "Is there anything to difficult for the Lord.
Let me be "blunt" here and say the Lord that physically appeared to Abraham was the "angel of the Lord" who is the pre-incarnate Jesus Christ. He first appears as the angel of the Lord at Genesis 16:7. He appears to Hagar and tells here He will multiply her seed and she shall bear a son. Genesis 16:7-12.
At vs13 Hagar says, "Then she called the name of the Lord who spoke to her, "Thou art a God who sees; for she said, Have I even remained alive here after seeing Him?" Vs15, "So Hagar bore a son and Abraham called him "Ishmael"
Now comes one of the interesting parts as far as I'm concerned. Genesis 17:1-3. "Now when Abram was ninety-nine years old, the Lord appeared to Abram and said to him, "I am God Almighty; Walk before Me, and be blameless, Vs2, And I will establish My covenant between Me and you, And I will multiply you exceedingly." Vs3, "And Abram fell on his face, and GOD talked with him, saying etc"
This was a physical/visible appearance of God to Abram and is verified at Genesis 17:22. "And when He/God finished talking with him, God went up from Abraham." Two things to notice here. One is the fact that I believe that the angel of the Lord who multiplied Hagar's descendants is the same being here at Genesis 17-1-3 that will multiply Abraham's descendants.
The second thing to notice is at Genesis 18 the Lord again physically appears to Abraham and God explains that Sarah will have a child who will be named Isaac. Getting back to your example at Genesis 1:1, notice what Genesis 18:33 says, "And as soon as He had finished speaking to Abraham the Lord departed; and Abraham returned to his place.
Genesis 19:1 says, "Now the TWO ANGELS came to Sodom on the evening as Lot was sitting in the gate of Sodom. These two angels are the same two men that appeared with the Lord at Genesis 18. So from Malachi 3:1 who do you think is the "messenger" of the covenant? I know for a fact it's surely not an actual angel.
