Increasing ISO increases electric current to increase exposure. Increasing electric current increases noise. Electricity causes noise. Noise in the .jpg may be reduced by the camera and by the noise reduction settings in the camera.

A cellphone will work just fine as long as what you want is within its capabilities.

What you need is the book "Mastering the Nikon D850" by Darrell Young.
Group-area AF mode gives 9 focus points to help prevent the camera from focusing on the background.
Using AF-C
and
Group-area AF
Focus on your subject using the back button (or press the shutter release button half way down) and continue to hold the button down as you track your subject.

Ther is a considerable amount of additional in formation on this mode as well as the others in "Mastering the Nikon D850". It is 656 pages long not including the preface or appendix and does an excellent job of explaining the D850.

I may not be answering the question that you asked but as a teacher (not photography) I find it best to have a good book to use when teaching. Usually go chapter by chapter, although sometimes the order can be changed.
Have the students study the material first, then in class lecture and and discuss the material that they have studied.

I am a bit confused about noise not related to ISO but exposure.
A quote from "The Manual of Photography".
"ISO settings in digital cameras essentially allow the user to trade noise for sensitivity. By increasing the ISO sensitivity, the camera shortens the exposure time and compensates this by and compensates this by increasing the levels of analogue and digital gain applied to the image. This increases the noise level in the image." Page 171.
If I understand this correctly it says noise is related to ISO.

To me it looks like a good case for bracketing, if your camera has that capability, then blending in post.

I shoot only Nikon cameras and lenses and always will. My equipment includes a Nikor 200-500, a D3300 and a D850. Be aware that the 200-500 is an FX lens. It will still give excellent pics on crop sensor. But no matter what you choose I recommend that you learn post processing.

I have seen in a store pictures hanging on the wall for sale and mostly printed on canvas. Supposedly my printer a Canon Pro 10 will print on canvas so I am wondering what type of canvas do I need, where to get it and how much does it cost? Also Matte paper was recommended to me and I have some, Canon gives away free paper when you order ink, but it leaves a very large border around the picture and it seems to use a lot of ink. Is there anyway around losing so much paper to the border? Also the price in this shop for the most expensive pictures is $250. frame included. The largest one is about 20 x 30, give or take, with a wood frame and matting. It looks like the frame costs a lot more than the picture.
I'm looking for clarification on how all this works.

Thank you;
Mark

I certanly am no expert but I will throw out a theory anyway. The smaller photo requires a smaller amount of comparisons and smaller numbers in its math problems resulting in better noise reduction.

Gotta be a Grateful Dead fan.