Looking For A New Camera

How do I know what to look for when buying a new camera?

Thanks for the ask to answer. What is refreshing about your question is that you have used your prior experience to guide your next steps.  Good for you.It isn't clear from your question whether or not the existing lenses are FX (i.e., full frame) compatible. Assuming they are, then your full frame choices are Nikon models D610 ($1,500), D750 ($1,900), D810 ($2,800), D4S ($5,550), and Df ($2,850) or to switch to a system where you can use your existing lenses via adapters, such as the Sony A7ii. Among the Nikon models, the D750 provides a significant upgrade to the D610 in the autofocus system and image processor. Both of these contribute significantly to low-light performance which is directly relevant to your interests in wedding photography.  The enhanced video features would be a help in that application as well. Of the others, the D4S is more of a sports shooter, and the Df is a little too quirky for what you have in mind. The D810 would be the obvious high-end choice, but it's not clear the additional resolution is really worth the cost for your interests.  Hope that helps.

I’m looking to buy a new camera and I have a budget of around $900. I can do $1000 but I’d rather not. What are good cameras and what are the specs of them?

The thing is you need not just a camera, but a camera + lens(es) to make pictures. Regardless of what camera you pick, your leftover budget will automatically force you to use low-grade consumer lenses, which are not that good optically. It really does not make sense to have a nice camera, if you can only put a crappy lens on it, unless your goal is to just learn how to use an SLR. To fully enjoy taking pictures with large-sensor cameras and be blown away by image quality, you would want to use pro lenses that cost over $1000 each.An alternative would be to use a high-end point & shoot camera with a fixed lens. But then you have to ask yourself whether your are gaining that much image quality compared to the loss of utility and compactness, vs. a smartphone camera.I would argue that for your budget, the best choice would be a high-end smartphone, such as iPhone XR or Huawei P20 Pro. The optics in smartphone cameras is pretty darn good for the budget, and the sophistication of the image processing to make pictures pretty blows away what is available in dedicated cameras. When you combine it with ultimate convenience (it easily fits in your pocket), and consider that you also get a phone and all kinds of other things with it, the package is hard to beat.

Looking for a new camera, what is mirror technology and how does it work?

Do you actually mean "mirror" technology, as in what is used in (D)SLRs and has been for the past 60 or so years? That's just a way to deflect light away from the film/sensor on to a glass screen so you can see an image of what the lens will project on to the film/sensor. When yon take the picture, the mirror quickly flips out of the way so the image is projected on to the film/sensor.On the other hand, if you mean mirrorless, that's just a way of saying the mirror mentioned above has been taken out and instead the lens projects its image directly on to the sensor all the time. To see what you're shooting, the screen or electronic viewfinder shows a live view of the sensor output. It's basically how point and shoot cameras have always worked, but the term "mirrorless" usually implies "interchangeable lens"; in other words you can use a variety of lenses much like you can with a (D)SLR.

I'm looking for a new camera to do mostly outdoor photography (of the Northern Lights, land, etc.) and would require a fisheye lens. What are some good cameras?

Pretty much any recent dSLR or mirrorless ILC, is going to do a pretty good job for you. Full frame is going to be a little bit better for northern lights and getting wide perspectives - but a lot of photography is not that... so don't be set on buying a full frame thinking is it going to take your photography to the next level.To be honest... I'm pretty sure your own ability is going to be more of limiting factor. I have a Nikon D610 (although I would recommend the D750 if you are going to buy)Obviously there is also the Canon 5DMkIII (this was taken at the same place a few minutes later)Really, whatever you can buy that is a good deal for your current budget is probably going to be fine.Here is a photo I found taken with an APS-C Nikon D7100

I'm looking to buy a new camera. Let's discuss options. I prefer Canon cameras. Looking into the 6D. Help?

I used to prefer Canon, but now I've moved on to Micro 4/3s and Sony's Mirrorless system.  They're far superior in practically every way unless you're doing actual sports which require a superior autofocus system (which is still hit and miss even when you know how to use the system)The 6D is definitely in the low end segment, but before purchasing, look into what's coming out soon.  The 6D Mark II is around the corner, the 5D mark IV is slated to come out in the fall as well, meaning prices of the 5D3 will plummet significantly.So back to the 6D, it's a full frame sensor, has a 9 point AF sensor, and it kinda sucks at video.  However pricing wise has it consistently below 1400 bucks.  For a beginner to intermediate user, it's a steal and a great way to step into the realm of Full Frame and continue using your prime 35mm 1.4L, 50mm 1.2L, 85mm 1.2L, and the 135mm 2L lenses.Don't have full frame lenses or L Primes?My question to you is, why bother sticking with Canon?

Looking for a new camera to replace my Nikon D7200 Digital SLR (24.2 MP, Wi-Fi, NFC), what would you recommend?

Nikon D500 Digital SLR Camera 20.9MP DX-Format On Ebay

I’m looking to purchase a new camera to be used for creating videos for YouTube. What are reasonably priced cameras on the market that will also produce quality content?

So much depends on what you are planning to shoot.For general outdoor daytime shooting in good light, a very inexpensive and modest camera will serve you very well. If you are looking at more demanding scenes like low light, or scenes with high contrast, then a more expensive camera will serve you better.However, your question suggests a common misconception among many, that only those with the most expensive cameras make good videos - and that is not true at all. In fact, if you are shooting yourself indoors, a modest camera with more money put into a good mic and needed lighting will give you far superior results than a dark room with a very pricey camera.So consider what and where you will be shooting. If you don’t have a good tripod, then get one. If you will be walking around holding your camera, then look into getting a gimbal or other device to stabilize the picture more. Nothing looks like shit more than horrible, shaky footage (look at the vacation videos on Youtube and there is no shortage of them). If shooting a lot in your own room or place, then concentrate on getting good lighting (only relying on the sun through windows often creates terrible backlighting, bad exposure, etc). Pick a good mic for shooting outside, or for you talking, a good lav mic.Make sure you get a camera with an external mic input. You could record audio separately and sync it up in post, but it can get to be a chore, and for many creators, really unnecessary. Many creators say that people are more tolerant of bad video than bad audio; something I have found to be true, at least to a point.If you want to make something with some long term viability and shooting something very photogenic, then consider getting something that can shoot in 4K. Prices for such gear have come down dramatically (the Lumix DMC-FZ300 is currently just $400) and even a modestly powerful computer can edit all the footage. There is a free powerful editor you can get - DaVinci Resolve.So think beyond the camera, and choose what works best for you.