I first saw "P Mode" being called "Professional Mode" in one of Ken Rockwell's columns.
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"Professional Exposure Mode

I almost always shoot in Professional exposure mode, the "P."

Today's Professional mode was originally called "Program" back in the 1970s by camera marketers, and this name still turns up in in some instruction manuals.

Pro photographers call it Professional mode, since that's what everyone uses. If we need different shutter speeds or apertures, all we do is flick the rear dial to get them.

Calling it Professional mode also helps steer newcomers the right way, since they are often working off old-wives' tales and trying to shoot Manual or Aperture-priority with no good reason."
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https://kenrockwell.com/tech/2010-06.htm (Scroll down about 1/3 of the way down)

That is the best advice you are going to get.

Everything you need to know about the Exposure Triangle is covered in the 1st chapter of Bryan Peterson's excellent book, "Understanding Exposure". The rest of the book goes into the depth of the subject and is a very enlightening and enjoyable read. Highly recommended for anyone seeking how to produce better photographs using the basics of the exposure triangle.

https://www.amazon.com/Understanding-Exposure-Fourth-Photographs-Camera/dp/1607748509/ref=sr_1_1?ie=UTF8&qid=1534513201&sr=8-1&keywords=understanding+exposure

Excellent photos.....again! Your Oshkosh photos, both on the ground and in the air, are just outstanding. Thanks for sharing them. I look forward to MORE if possible.

Yes, I have experienced the same thing with a Nikon D3300 and a Nikor 16-85mm lens. Autofocus became erratic and I noticed the battery level was down to 18%. Replaced the battery with a full charged one and the problem disappeared instantly. I'll pay more attention to my battery charge level in the future.

Excellent photography. Just EXCELLENT.

My daughter-in-law gave me a Tamron 18-270 (Nikon mount) about a year ago. I love it. Relatively small and compact, it makes a great walk-around lens. It gets excellent reviews and is priced right. It comes with a 5-year warranty but YOU MUST keep your store receipt showing your purchase information to register it for the warranty. I tried to register mine to get the 5-year warranty but I didn't have the sales receipt because it was a gift and my daughter-in-law doesn't save receipts. SO KEEP YOUR PROOF OF PURCHASE.

This thread reminds me why I always buy B&W filters that have a BRASS mounting ring. Brass mounting rings are less likely to seize up if screwed on too tightly.

Thanks for a little bit of humor to help me wake up this morning.

That would be almost impossible. Best Buy is not a camera store, it's a store that sells cameras. Too many times I have asked the salesman in the photography section a simple question about various cameras or lenses. Too many times the response has been something like "Oh, I don't work here. I work in appliances. I'm just filling in for Jack who didn't come in today." The best advice for shopping at Best Buy is "LET THE BUYER BEWARE". The buyer in the OP's case went in and bought the camera without doing her homework first. It's hard to blame the salesman in this case. Her D7200 lenses will work on her D850. They just won't perform to the full potential of her camera.

Check out the iPhone 6S at your local Apple store. It has most of the features of the "latest and best" models and costs around $300.

I bought a Platinum UV at Best Buy and then read the reviews from reviewers that I trust. I took it back the next day and exchanged it for a Hoya UV that had excellent reviews. Now I buy B&W filters from B&H Photo. They are excellent filters at a reasonable price. I also don't shop at Best Buy anymore after a couple of bad experiences with them.

I have too many cameras now but my next one will be the Fujifilm X-T2. It's a bit larger and a bit more expensive than the X-T20 but it is the perfect size for me. I have handled the camera in the stores and I like the retro setup...having all the basic settings right on top of the camera without going into the menus like my Sony requires. I've read numerous reviews on the Fuji line and the quality is just excellent. The optics provide excellent results and as one reviewer on this forum (I think it was Cat Marley) has said, The camera is just "elegant". I will start with the 18-55 kit lens and go from there. I already feel a gas attack coming on.

Thanks for posting what I was thinking.

Though I love B&W photography, in this case I prefer the color rendition by far. Beautiful photo of a beautiful lady.