My information is a bit dated and limited but here it is.
We flew into Bangkok did an overnight at the airport Intercontinental and flew to Seim Reap the next day. This was April 2019.
Easy flight, 55 minutes. Siem Reap airport is small and easy to navigate.
One runway, open air terminal, the only crowd will be from your plane.
At that time most travel sites recommended getting your visa upon arrival at Siem Reap which we did.
Back then it was $30/person, US dollars preferred payment method, and you will find that the dollar is widely accepted in Cambodia.
I don’t recall the rest of the visa details but we flew back to Bangkok after 3 days, embarked Viking Orion and stopped in Sihanoukville a few days later. No additional visa was required.
Enjoy your adventure, Angkor Wat and surrounding area was incredible.
BTW, we stayed in Park Hyatt in central Siem Reap, highly recommended.