The difference between the Nikon D750 and D850 happens to be one of the most considerable and well defined product differentiation achieved in modern photography :~)
The D850 is a seminal state of the art camera that internally is identical to the contemporary flagship the D5 in every way except for major improvements like high resolution and exceptional sensor which is arguably not surpassed almost six years later. Whilst the D750 is a cost engineering marvel. The D750 is a very fine camera, but nothing in the league of the D850.
If you are actually considering purchasing a D750 I'd strongly recommend that you investigate the D780 which only appears to be much the same, but includes the majority of the successor mirrorless technology developments including excellent video giving live view operating far better than any other DSLRS from Nikon including the flagships.
Nikon managed some exemplary product differentiation in the last range of DSLR cameras. Unfortunately for everyone Nikon can't seem to be anything like as clear and focussed with the Z series mirrorless range.
Edit: the D850 compared with the contemporary flagship D5, is capable of only slower rates of shooting, has a smaller battery, no extended grip and dual orientation controls and the sensor can't -quite- perform in low light like a D5. In every non sports shooting environment the D850 would be regarded as a flagship, but for historical reasons and the attachment rate of D3/4/5/6 users buying proportionately many more elite lenses, it's the lower resolution action and low light big pro cameras that get the flagship accolade. Single digit pro cameras also optimize photo processing in camera for JPEG output as is big agency wont and urgency the need for finished product out of the box without any processing. In case you're looking into this, JPEG output from the D5/6 is incredibly good. "Merely" 20MP sensors in these cameras are far better than any specification can indicate, being developed to produce the cleanest pixel level signal, which really is visible in the output. A hardly used very good condition D5 can be had for $1500 lenses dumped en masse for Z upgrades are bargains. If you can deal with the bulk of these cameras, you'll very unlikely i've dissatisfied. And de facto unbeaten autofocus with a major feature omitted from new designs Group Priority AF which glues the focus to your selected nearest subject without any deviation that happens in the Z bodies because of the omission of this professional feature. Novices are apparently buying the Z9 mirrorless flagship because it's not got AF compromises to frustrating degree. But you can have a complete kit for the same money in the DSLR line.