Welcome to the GPF! A questions for you -
You say "koi pond" - is that what you really mean? Like is the purpose of your pond to raise big, beautiful koi with no plants and crystal clear water? Or do you want an eco-style pond with fish, plants, rocks, etc?
How big do you plan for this pond to be? If you've read anything here at all, you know "koi" means "gallons" - lots and lots of water volume required for big fish. Does 1000 gallons sound like a lot? It's not.
Answers to those two questions will answer a lot of your remaining questions.
Can you have a bottom drain and a bog filter - yes. Check out
@GBBUDD and his build thread. Do you need a bottom drain? Well, maybe. See question #1. Do you want a bottom drain? See question #1. Many factors to consider when choosing to add a bottom drain or not.
Do you need a skimmer? Not necessarily. Research vanishing edge, negative edge or intake bay builds.
Do you need a UV - my simple answer is "no". If you understand what a UV light does, you may decide you would never want one. Plus it's one more thing to maintain.
That first question though is where you start - what kind of pond do I want? The term "koi pond" gets thrown around much like "Kleenex" or "Clorox". Knowing where you want to end up will help you decide where to begin.
There are LOADS of great pond build threads here, and, as
@GBBUDD said - YouTube is a fantastic resource. These professional pond builders are giving away ALL their secrets to building ponds that are both beautiful and function well. Take advantage of what they're offering. If you can do it yourself, you'll save yourself a ton of money and have a whole lot of pride in yourself and your creation when it's all said and done.
Stick around here, ask questions, read read read... you'll get all the help you're willing to accept and some advice that you may wish you didn't have to hear. Post some photos of where you plan to locate your pond and start with that. There are definitely right and wrong places to build a pond in a yard.