Abiogenesis happened. There was no life on earth 20 billion years ago. There is life on earth today. Therefore life appeared on earth some time in the last 20 billion years.Abiogenesis is a joke. Even if you did have all the materials needed to make a "building," you still need builders. No building has ever made itself and it is asinine to suggest otherwise.
The assembling the materials was done by chemistry. I suggest you start by learning about lipid bilayers.
Because we have rocks dating from the early earth which tell us about the chemical composition of the atmosphere and the seas. Observations of other planets in the universe also provided some information.And how do you know what the conditions of the early earth were?
Something you need to learn about science is that any "How do you know ..." question will always have an answer. Science always has evidence to back up its statements. If there is no evidence then science says, "We don't know."
Stellar nucleosynthesis for the most part, though some of the hydrogen could date back to the Big Bang.Where did those chemicals come from?
Correct. Just as the fact that the computer program was designed does not mean that the process it emulated was designed.It means that the painting of God was designed.