What's the ideal calorie deficit to lose fat and gain muscle?

I don't know that there's a specific number we can point to with certainty but in general, the smaller the deficit, the better your chance of being able to build muscle in a deficit. I would recommend keeping the deficit below 500 kcal. 300 sounds like a good starting point. Don't forget to get plenty of protein and, of course, engage in a progressive resistance program.

Losing fat while build muscle is a theoretic possibility, no doubt about it and can be done but if you have an appreciable amount of fat to lose and you want to build muscle during this time, well imo, that's a fools errand and very frustrating trying to see any appreciable fat lose and muscle gain week after week, simply because of the narrow calorie deficits required and the inaccuracy of counting calories in general. This is more suited for people that have a smaller mount of body fat and make the decision to reduce body fat lower while maintaining or hopefully building muscle or minimizing fat gain while dedicating their diet to increasing muscle mass and is basically what is referred to as body recomposition, which is what I'm doing at the moment but

If your looking to lose a decent amount of body fat and from what you've divulged I'd say that you probably do, then imo, I'd recommend that you concentrated on fat lose primarily while minimizing muscle loss, which is more about diet and weight training with a decent amount of quality protein while endorsing a decent progressive overload weight regime and when your down to a more desirable overall weight then take on the exacting and surgical attempts at body recomposition, sorry for the descriptors, but that's how it feels at the moment trying to get to my goal with mm current recomp. Cheers and good luck with whatever you decide.

Yep, if you are a beginner lifter and/or have a lot of fat to lose you can certainly do both at the same time. A 5'9 male that weighs 200 probably means you do not have a crazy amount of fat to lose.

Bottom line... Prioritize one thing. Eat and train accordingly until you reach your goal in that area. Then prioritize the next thing. Otherwise you will likely spin your wheels and not make much progress with losing weight OR gaining muscle. I speak from experience.

I'm 5'9" (same as OP) and weigh 190, and will say that I do not have 40-50lbs of fat I can lose.

Think about this, @junkmail4379. If you focus on losing fat while maintaining the muscle you already have, you'll appear more muscular as the fat hiding it is stripped away. An old personal trainer once told me you can get a better shoulders/waist ratio by either losing fat around the middle or gaining muscle up top, since it's often the ratio which is visually appealing more so than the actual amount of muscle itself. Keep the protein up, weight train, and cut calories by 300-500 to shed some fat away, and you'll LOOK more muscular. Then once you get down to your desired weight, you can reverse course and add 300-500 calories, still with protein and weights, to increase muscle further.

Steven Jackson, a former running back for the (then) St Louis Rams, was 6'2" by 238 lbs. BMI-wise he was obese, but with single digit body fat he was simply a beast.