You tell them where you're going to stay. You cant get a visa without showing your flight and lodging details. But you have to stay in a specific type of hotel that Thailand wants you to - one of the ones listed on the asq.in.th directory.
ATK/Rapid-Result tests are less expensive to perform than PCR tests. Also test price is dependent on your country...I'm from the US and the mandatory pre-flight PCR test is free...