Too much to cover in one thread, but congrats for having the sutterbug illness..
In reality, just shoot RAW.. you can tweak your exposure and WB to suite. YMMV.
If your running Windows 10, you can likely see the thumbnail preview of the RAW file.
If not, you CAN download and install the DNG CODEC to your Windows OS and view
See last link in the forum thread, this is the one I downloaded and installed:
Affinity Photo is $50, I have switched away from PS to Affinity, it allows me to do HDR, lots of tweaking etc..
without spending a lot of extra money. Also, it supports PS plugins.. you can download the NIK filters
from the Google version and use them in Affinity to really add some artistic spark! NIK/Google filters
are *free*. (There are a boat load of Affinity tutorials on Youtube)
YMMV, but I give up on manufacturer's RAW files, I convert everything to DNG now and run off that,
Affinity (and almost every software worth a hoot) reads DNG just fine, I dont loose any of the data
that I care about nor any of the dynamic range. DNG is just simpler to live with. YMMV.
Some light reading here:
DNG converter is free from Adobe:
Your original question: Your ISO is set to AUTO but always shoots at 400.
I will yield to a Cannonite to give specific guidance but if you are shooting manual and
specifying aperture and shutter speed with AUTO ISO, the ISO should adjust
depending on your light.
With my D7500, I do this regularly and have been VERY happy with the results!
Sometimes you want a little bit of auto, but not with shutter or aperture, at this
time the AUTO ISO is a great tool!
If I only helped a little, I did my duty..